If upset, act like a dog! (4)
I believe the answer is:
fido
'dog' is the definition.
(I know that Fido is a dog)
'if upset act' is the wordplay.
'upset' is a reversal indicator.
'act' becomes 'do' (to do is to act or execute a task).
'if' back-to-front is 'fi'.
'fi'+'do'='FIDO'
'like a' acts as a link.
(Other definitions for fido that I've seen before include "it would disperse fog" , "Fog disperser" , "Dog perhaps" , "Typical dog's name" .)