One biscuit or a few mixed with the first of the rusks (5)
I believe the answer is:
wafer
'one biscuit' is the definition.
(wafer is a kind of biscuit)
'a few mixed with the first of the rusks' is the wordplay.
'mixed with' indicates an anagram.
'the first of' says to take the initial letters.
The first letter of 'rusks' is 'r'.
'a'+'few'='afew'
'afew' with its letters mixed up with 'r' can make 'WAFER'.
'or' acts as a link.
(Other definitions for wafer that I've seen before include "Ice-cream biscuit" , "Communion bread" , "Light thin biscuit" , "Thin, crisp biscuit or bread" , "Very thin 14 Down" .)