Party drug makes head go blue initially (4)
I believe the answer is:
bash
'party' is the definition.
(term for a party or celebration)
'drug makes head go blue initially' is the wordplay.
I cannot quite see how this works, but
'drug' could be 'h' (H is an example) and 'h' is present in the answer.
The remaining letters 'bas' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bash that I've seen before include "Party - whack" , "Go" , "Party - punch" , "Heavy blow; party" , "Party; beat" .)