Superficial former partner learnt to change (8)
I believe the answer is:
external
'superficial' is the definition.
(I've seen this before)
'former partner learnt to change' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'to change' indicates an anagram.
'learnt' is an anagram of 'ternal'.
'ex'+'ternal'='EXTERNAL'
(Other definitions for external that I've seen before include "Lying outside" , "Superficial" , "involving foreign nations" , "Relating to the outside" , "Outward" .)