Northern spouse is missing her very much (3,4)
I believe the answer is:
not half
'very much' is the definition.
(I've seen this before)
'northern spouse is missing her' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'spouse' becomes 'otherhalf' (informal term for a partner or spouse).
'is missing' suggests deleting specific letters.
'otherhalf' with 'her' removed is 'othalf'.
'n'+'othalf'='NOT HALF'
(Other definitions for not half that I've seen before include "Rather! - less than 50%?" , "less than fifty per cent" , "thoroughly" , "Forty nine per cent" , "Very much, exceedingly" .)