Account for what is no longer clear (7)
I believe the answer is:
explain
'account for' is the definition.
(I've seen this before)
'no longer clear' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'clear' becomes 'plain' (synonyms).
'ex'+'plain'='EXPLAIN'
'what is' is the link.
(Other definitions for explain that I've seen before include "Make understandable" , "Defend" , "Resolve" , "Make comprehensible" , "Elucidate, make clear" .)