Wife in pub used to slim? (3-3)
I believe the answer is:
low-cal
'used to slim?' is the definition.
(low in calories, used for slimming)
'wife in pub' is the wordplay.
'wife' becomes 'w' (abbreviation).
'in' is an insertion indicator.
'pub' becomes 'local' (one's local pub).
'w' placed within 'local' is 'LOW-CAL'.
(Other definitions for low-cal that I've seen before include "Less fattening" , "With reduced food energy" .)