A very thin material is under consideration (2,5)
I believe the answer is:
at issue
'under consideration' is the definition.
(synonyms)
'a very thin material' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'material' could be 'tissue' (tissue is a kind of material) and 'tissue' is found within the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for at issue that I've seen before include "Questionable" , "in dispute" , "Being discussed or disputed" , "In question, under discussion" , "under debate" .)