What is a word for "not well studied"

I've seen variations on the following:

• little studied
• under-examined
• open field
• unexplored

These are often combined with relatively so the reviewers don't carp.

If you are asking for single words, "cloudy", "nebulous", "blear", "obscure"

blear - obscure to the view or imagination Merriam-Webster

nebulous - unclear, vague, or ill-defined OED

cloudy - uncertain; nuclear OED

If you are asking for phrases, then you have quite a few good choices

• not completely understood.
• not fully elucidated.
• needing further research.
• not fully clarified.

Examples from scientific papers:

• "Exact mechanisms responsible for this kind of endocrinopathy have not been completely clarified." Autoimmune Diseases
• "The mechanism of pain has not yet been fully understood."
• "The mechanism of cerebral uptake has not been completely elucidated." Nuclear Medicine
• "Its exact mechanism of action needs further research."

More than one word comes to mind, but I am not sure if they are what you're looking for. From the world of medicine, "of unknown etiology" and "idiopathic," http://www.medicinenet.com/script/main/art.asp?articlekey=3892. From literature and reviewing, recherche, with an accent over the final e, http://www.merriam-webster.com/dictionary/recherch%C3%A9

Author I'm working with (ELL) used "understudied" but I'm not sure that's a good idea since "understudy" is well known with a completely unrelated meaning.

Understudied (adj): An understudied subject has not been studied as much as it should be or as much as you would expect.

Example: The terrapin is a surprisingly understudied species.
[Cambridge English Dictionary]

There isn't a single word for "not well studied", but there are adverbs such as little, poorly and relatively that are more commonly used in place of “not well”.

• little-known / little known
• relatively obscure
• poorly studied
• underresearched / under-researched

Cystic fibrosis: how to recognize this little-known illness

In short, this relatively obscure plant is far more interesting and scientifically valuable than anyone might have guessed.

Institutional racism is shown to account for significant continuities in the approach of universities to race equality, yet race in higher education is an under-researched area.

Ecological implications of recently discovered and poorly studied sources of energy for the growth of true fungi especially in extreme environments