Clarify what's no longer clear (7)
I believe the answer is:
explain
'clarify' is the definition.
(thesaurus)
'no longer clear' is the wordplay.
'no longer' becomes 'ex'.
'clear' becomes 'plain' ('plain' can be similar in meaning to 'clear'**).
'ex'+'plain'='EXPLAIN'
'what's' is the link.
(Other definitions for explain that I've seen before include "Pin axle (anag.)" , "Interpret" , "Defend" , "Shed light" , "Make intelligible and account for" .)