Muggins heading off to join forces (4)
I believe the answer is:
ally
'join forces' is the definition.
(to ally with someone is to join forces with them)
'muggins heading off' is the wordplay.
'muggins' becomes 'wally' (both can mean a foolish person).
'heading off' means to remove the first letter.
'wally' with its initial letter removed is 'ALLY'.
'to' is the link.
(Other definitions for ally that I've seen before include "He fights on your side" , "Friendly nation" , "Associate that provides assistance" , "Friend, partner" , "Join forces with" .)