Describe old lover, not much to look at (7)
I believe the answer is:
explain
'describe' is the definition.
(I've seen this before)
'old lover not much to look at' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'not much to look at' becomes 'plain' (both can mean dull or unattractive).
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Defend" , "Give reason for" , "Spell out" , "Elucidate - define" , "Shed light" .)