Serpent starts to act strangely perhaps (3)
I believe the answer is:
asp
'serpent' is the definition.
(asp is a kind of serpent)
'starts to act strangely perhaps' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'act strangely perhaps' is 'ASP'.
(Other definitions for asp that I've seen before include "snake appears" , "Cleopatra's supposed nemesis" , "Venomous snake" , "Snake seen in a spa, strangely" , "something at the zoo" .)