Thin layer impaired by lines at first (7)
I believe the answer is:
lamella
'thin layer' is the definition.
(I've seen this in another clue)
'impaired by lines at first' is the wordplay.
'impaired by' becomes 'lame' (I can't explain this - if you can you should believe this answer much more).
'lines' becomes 'll' (an abbreviation for line written twice).
'at first' becomes 'a' (lst letter of 'at').
'lame'+'ll'+'a'='LAMELLA'
Can you help me to learn more?
(Another definition for lamella that I've seen is " Thin plate".)