No longer obvious what one must do to make things clear (7)
I believe the answer is:
explain
'what one must do to make things clear' is the definition.
(to explain is to clarify something)
'no longer obvious' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'obvious' becomes 'plain' (similar in meaning).
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Give reason for" , "offer clarification" , "Elucidate - define" , "Elucidate, make clear" , "Tell us how" .)