Small fish is upset (5)
I believe the answer is:
shake
'upset' is the definition.
(I've seen this before)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'hake' (I've seen this before).
's'+'hake'='SHAKE'
'is' acts as a link.
(Other definitions for shake that I've seen before include "Frothy milk drink" , "Tremble, vibrate" , "wave" , "Vibrate; drink" , "Shiver" .)