Wife comes in behind Labour (5)
I believe the answer is:
sweat
'labour' is the definition.
(sweat is a kind of labour)
'wife comes in behind' is the wordplay.
'wife' becomes 'w'.
'comes in' is an insertion indicator.
'behind' becomes 'seat' (synonyms).
'w' going inside 'seat' is 'SWEAT'.
(Other definitions for sweat that I've seen before include "Trouble in Australia" , "It's hard work" , "Perspire" , "Salty fluid secreted by the body" , "Waste (anag.)" .)