Observing husband in void (7)
I believe the answer is:
nothing
'void' is the definition.
(I have seen 'A void' mean 'nothing' so perhaps 'void' could also mean 'nothing')
'observing husband in' is the wordplay.
'observing' becomes 'noting' (note can mean to observe or notice).
'husband' becomes 'h' (abbreviation).
'in' means one lot of letters goes inside another.
'noting' enclosing 'h' is 'NOTHING'.
(Other definitions for nothing that I've seen before include "Unimportant item" , "what House of Peers did particularly well?" , "Naught, nil, nix" , "Item of no importance" , "Zero, nil" .)