Rabbit punch lands on British head of state (6)
I believe the answer is:
jabber
'rabbit' is the definition.
(I know that rabbit can be written as jabber)
'punch lands on british head of state' is the wordplay.
'punch' becomes 'jab' (jab is a kind of punch).
'lands on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'head of state' becomes 'ER' (abbreviation for Elizabeth Regina).
'jab'+'b'+'er'='JABBER'
(Other definitions for jabber that I've seen before include "Indistinct talk" , "poker?" , "Rapid (foreign) talk" , "one that's given punch" , "Talk fast like a monkey" .)