Observing husband going in for a trifle (7)
I believe the answer is:
nothing
'trifle' is the definition.
(thesaurus)
'observing husband going in' is the wordplay.
'observing' becomes 'noting' ('note' can be a synonym of 'observe').
'husband going' becomes 'h' (I've seen this in other clues).
'in' is an insertion indicator.
'noting' going around 'h' is 'NOTHING'.
'for a' acts as a link.
(Other definitions for nothing that I've seen before include "What's in a vacuum?" , "Nought; of no value" , "Item of no importance" , "unimportant or > unintelligible" , "A void" .)