Counterpart merely returning washing machine (4,3)
I believe the answer is:
twin tub
'washing machine' is the definition.
(I've seen this before)
'counterpart merely returning' is the wordplay.
'counterpart' becomes 'twin' (synonyms).
'merely' becomes 'but' ('but' can mean 'only' or 'merely').
'returning' shows that the letters should be reversed in order.
'but' backwards is 'tub'.
'twin'+'tub'='TWIN-TUB'
(Other definitions for twin tub that I've seen before include "Washing machine" , "Cleaner" , "With extra drum" .)