Screen short, first class adaptation at last (7)
I believe the answer is:
curtain
'screen' is the definition.
(I know that curtain is a type of screen)
'short first class adaptation at last' is the wordplay.
'short' becomes 'curt' (synonyms).
'first class' becomes 'ai' (Ac - top quality).
'at last' says to take the final letters.
The last letter of 'adaptation' is 'n'.
'curt'+'ai'+'n'='CURTAIN'
(Other definitions for curtain that I've seen before include "Hanging cloth" , "Cover for window etc" , "It's raised before the start of a play" , "Stage concealer" , "Hanging drapery for window" .)