Hiss: 'Turn over if snoring!' (4)
I believe the answer is:
fizz
'hiss' is the definition.
(thesaurus)
'turn over if snoring' is the wordplay.
'turn over' is a reversal indicator.
'snoring' becomes 'zz'.
'if' back-to-front is 'fi'.
'fi'+'zz'='FIZZ'
(Other definitions for fizz that I've seen before include "Bubbles in a drink" , "Gas in drink" , "Effervescent drink" , "Champagne, say?" , "Effervescence" .)