Cook slowly, throwing in a bit of lard? Less fat (7)
I believe the answer is:
slimmer
'less fat' is the definition.
'cook slowly throwing in a bit of lard?' is the wordplay.
'cook slowly' becomes 'simmer'.
'throwing in' means one lot of letters goes inside another.
'a bit of' suggests taking the first letters.
The first letter of 'lard' is 'l'.
'simmer' enclosing 'l' is 'SLIMMER'.
(Other definitions for slimmer that I've seen before include "in better shape?" , "More slender" , "More slenderly built" , "person who's trying to lose weight" , "thinner" .)