For slimmers, food always there for starters, costing nothing (3-4)
I believe the answer is:
fat-free
'for slimmers' is the definition.
(I've seen this before)
'food always there for starters costing nothing' is the wordplay.
'for starters' says to take the initial letters.
'costing nothing' becomes 'free' (given away for free).
The first letters of 'food always there' is 'fat'.
'fat'+'free'='FAT-FREE'
(Other definitions for fat-free that I've seen before include "Containing no greasy substances" , "Very low in saturates/unsaturates" , "Without solid animal or vegetable oils" , "Like some restricted diets" , "Lite" .)