Small fish tremble (5)
I believe the answer is:
shake
'tremble' is the definition.
(I know that tremble can be written as shake)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'hake' (type of fish).
's'+'hake'='SHAKE'
(Other definitions for shake that I've seen before include "Tremble; milk drink" , "Shiver" , "Tremble, vibrate" , "Frothy milk drink" , "Upset" .)