Correspond after a fourth letter? OK (8)
I believe the answer is:
adequate
'ok' is the definition.
(thesaurus)
'correspond after a fourth letter?' is the wordplay.
'correspond' becomes 'equate' ('equate' can be a synonym of 'correspond').
'after' is a charade indicator (letters next to each other).
'fourth letter?' becomes 'd' (fourth letter of the alphabet).
'a'+'d'='ad'
'equate' after 'ad' is 'ADEQUATE'.
(Other definitions for adequate that I've seen before include "Barely sufficient" , "Sufficient, good enough" , "Just about sufficient, acceptable" , "Fair to middling" , "Sufficient or all right" .)