Catch spouse scratching backside in mirror (6)
I believe the answer is:
entrap
'catch' is the definition.
(to entrap is to catch with a trap)
'spouse scratching backside in mirror' is the wordplay.
'spouse' becomes 'partner' (synonyms).
'scratching backside' means to remove the last letter (the back of the letter is erased or scratched).
'in mirror' says the letters should be written in reverse.
'partner' with its last letter taken away is 'partne'.
'partne' written backwards gives 'ENTRAP'.
(Other definitions for entrap that I've seen before include "Ensnare, catch" , "Lure into wrongdoing in order to arrest" , "Snare, trick" , "Trick or beguile for parent" , "Something to do with gin" .)