Clear up for former partner, unattractive (7)
I believe the answer is:
explain
'clear up' is the definition.
(to clear up is to explain or clarify)
'former partner unattractive' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'unattractive' becomes 'plain' (similar in meaning).
'ex'+'plain'='EXPLAIN'
'for' acts as a link.
(Other definitions for explain that I've seen before include "Give reason for" , "Make understandable" , "area affected by mountain formation?" , "Make comprehensible" , "Pin axle (anag.)" .)