Tremble like a small fish (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' (hake is a kind of fish).
's'+'hake'='SHAKE'
'like a' is the link.
(Other definitions for shake that I've seen before include "Frothy milk drink" , "Jolt or agitate" , "Rattle" , "rock" , "Shiver" .)