Break a small sleep (4)
I believe the answer is:
snap
'break' is the definition.
(I know that snap is a type of break)
'a small sleep' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'sleep' becomes 'nap' (a nap is a short sleep).
's'+'nap'='SNAP'
(Other definitions for snap that I've seen before include "Break sharply like dry twig" , "Spell of cold weather and card game" , "Break; liveliness" , "Picture" , "Speak irritably; photo" .)