Describe former flat (7)
I believe the answer is:
explain
'describe' is the definition.
(I've seen this before)
'former flat' is the wordplay.
'former' becomes 'ex'.
'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" , "Describe" , "Tell us how" , "Elucidate, make clear" , "Resolve" .)