Former partner given wrong milk (6)
I believe the answer is:
extort
'milk' is the definition.
(I've seen this before)
'former partner given wrong' is the wordplay.
'former partner' becomes 'ex'.
'given' means one lot of letters go next to another.
'wrong' becomes 'tort' (legal term).
'ex'+'tort'='EXTORT'
(Other definitions for extort that I've seen before include "Compel to pay" , "Obtain money with menaces" , "To bleed" , "Blackmail" , "Get through threatening" .)