Fragment is small certainly (6)
I believe the answer is:
snatch
'fragment' is the definition.
(as in hearing a snatch of a song)
'small certainly' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'certainly' becomes 'natch' (I've seen this in another clue).
's'+'natch'='SNATCH'
'is' acts as a link.
(Other definitions for snatch that I've seen before include "Piece of music" , "heist" , "bag" , "pull" , "Deftly seize" .)