Small sleep break (4)
I believe the answer is:
snap
'break' is the definition.
(snapping is a kind of breaking)
'small sleep' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sleep' becomes 'nap' (I've seen this before**).
's'+'nap'='SNAP'
(Other definitions for snap that I've seen before include "Become unhinged" , "Break sharply like dry twig" , "Simple card game - gotcha!" , "Bite at or utter angry words" , "Pans out in simple card game" .)