Partner improved, to an extent (6,4)
I believe the answer is:
better half
'partner' is the definition.
(I've seen this before)
'improved to an extent' is the wordplay.
'improved' becomes 'better' ('better' can be similar in meaning to 'improved').
'to an extent' becomes 'half'.
'better'+'half'='BETTER HALF'
(Other definitions for better half that I've seen before include "partner" , "Spouse (informal)" , "Husband" , "Wife (colloq.)" , "One's spouse" .)