Unusually small bit players (6)
I believe the answer is:
extras
'bit players' is the definition.
(I've seen this before)
'unusually small' is the wordplay.
'unusually' becomes 'extra' (extra can mean to an unusual degree).
'small' becomes 's' (abbreviation - e.g. clothes size).
'extra'+'s'='EXTRAS'
(Other definitions for extras that I've seen before include "minor players?" , "Sundries" , "Additions" , "Runs not scored from the bat" , "Inessentials" .)