Party remains behind Boris, initially (4)
I believe the answer is:
bash
'party' is the definition.
(term for a party or celebration)
'remains behind boris initially' is the wordplay.
'remains' becomes 'ash' (remains of a fire).
'behind' means one lot of letters go next to another.
'initially' suggests taking the first letters.
The initial letter of 'boris' is 'b'.
'ash' put after 'b' is 'BASH'.
(Other definitions for bash that I've seen before include "Smite; a party" , "social occasion" , "Party - clobber" , "crash > party" , "Party; attempt (slang)" .)