Disturb small fish (5)
I believe the answer is:
shake
'disturb' is the definition.
(thesaurus)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'hake' (I've seen this before).
's'+'hake'='SHAKE'
(Other definitions for shake that I've seen before include "Flavoured milk drink" , "Upset" , "Tremble, quake" , "Jolt or agitate" , "damaging blow" .)