It must exist within snack (4)
I believe the answer is:
bite
'snack' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for bite that I've seen before include "Get teeth into" , "Take effect; snack" , "(Of fish) take the bait" , "Mouthful" , "Punch" .)