Snapper with good roll used up (5)
I believe the answer is:
gator
'snapper' is the definition.
(short for alligator, an animal which 'snaps')
'with good roll used up' is the wordplay.
'with' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'roll' becomes 'rota' (rota is a kind of roll).
'used up' is a reversal indicator (in a down clue, letters go up).
'rota' back-to-front is 'ator'.
'g' next to 'ator' is 'GATOR'.
(Other definitions for gator that I've seen before include "One taking snaps" , "one snaps in America" , "beast?" , "Short reptile" , "Informal American term for semi-aquatic reptile" .)