Couples start union up (6)
I believe the answer is:
unites
'couples' is the definition.
(I've seen this in another clue)
'start union up' is the wordplay.
'start' becomes 'setin' (setting in is a kind of starting).
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'setin'+'u'='setinu'
'setinu' written backwards gives 'UNITES'.
(Other definitions for unites that I've seen before include "Becomes one" , "(old) money?" , "Combines" , "Comes together for a common purpose" , "Joins together as one" .)