Clear up former flat (7)
I believe the answer is:
explain
'clear up' is the definition.
(to clear up is to explain or clarify)
'former flat' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'flat' becomes 'plain' (I've seen this in other clues).
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Spell out" , "Elucidate - define" , "Resolve" , "Shed light" , "Elucidate, clarify" .)