Merciless wife gets worker working (6)
I believe the answer is:
wanton
'merciless' is the definition.
(thesaurus)
'wife gets worker working' is the wordplay.
'wife' becomes 'w' (abbreviation).
'gets' means one lot of letters go next to another.
'worker' becomes 'ant' (worker is a type of ant).
'working' becomes 'on' (e.g. a machine is working or on).
'w'+'ant'+'on'='WANTON'
(Other definitions for wanton that I've seen before include "Irresponsible, lacking proper restraint" , "Libertine" , "hussy" , "Reckless" , "uncalled-for" .)