Counter happening to pass 100 (4)
I believe the answer is:
chip
'counter' is the definition.
(chip is a kind of counter)
'happening to pass 100' is the wordplay.
'happening' becomes 'hip' (both can mean cool or fashionable).
'to pass' means one lot of letters go next to another.
'h00' becomes 'C' (roman numeral).
'hip' put after 'c' is 'CHIP'.
(Other definitions for chip that I've seen before include "Token - golf shot - potato piece" , "Piece of potato (found on someone's shoulder?)" , "Pare" , "Piece of wood, potato, or silicon" , "Piece off block (or on shoulder?)" .)