Chew small amount, with energy (4)
I believe the answer is:
bite
'chew' is the definition.
(thesaurus)
'small amount with energy' is the wordplay.
'small amount' becomes 'bit' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'bit'+'e'='BITE'
(Other definitions for bite that I've seen before include "Punch" , "Morsel" , "Small amount of food" , "Quick snack" , "champ" .)