Snake less happy after losing its head! (5)
I believe the answer is:
adder
'snake' is the definition.
(type of snake)
'less happy after losing its head' is the wordplay.
'less happy' becomes 'sadder' ('sad' can be the opposite of 'happy' so perhaps 'sadder' is reasonable for 'less happy').
'after losing its head' means to remove the first letter.
'sadder' with its initial letter removed is 'ADDER'.
(Other definitions for adder that I've seen before include "Reptilian" , "An arithmetic snake" , "accountant?" , "Type of snake" , "Venemous snake" .)