Clear up for former partner, not attractive (7)
I believe the answer is:
explain
'clear up' is the definition.
(to clear up is to explain or clarify)
'former partner not attractive' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'not attractive' becomes 'plain' (I've seen this before).
'ex'+'plain'='EXPLAIN'
'for' acts as a link.
(Other definitions for explain that I've seen before include "Pin axle (anag.)" , "Describe" , "Defend" , "Show the whys and wherefores" , "Expound" .)