Chewed on small pieces (4)
I believe the answer is:
bits
'pieces' is the definition.
(bit is a kind of piece)
'chewed on small' is the wordplay.
'chewed' becomes 'bit'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'small' becomes 's' (abbreviation - e.g. clothes size).
'bit'+'s'='BITS'
(Other definitions for bits that I've seen before include "Small units of digital memory" , "Pieces, fragments" , "Small pieces" , "Parts" .)