Brief sleep (3)
'brief sleep' is the definition.
(a nap is a short sleep; 'brief' suggests an abbreviation)
This is all the clue.
(Other definitions for nap that I've seen before include "Short raised fibres on cloth on snooker table" , "Raised fibres on surface of cloth or leather" , "Short sleep, card-game" , "Sleep - card game - fibres on a snooker table" , "A short sleep or snooze" .)
(Other definitions for snooze that I've seen before include "Quick nap" , "Take a nap" , "Doze, nap" , "A brief nap" , "An informal nap" .)