Join King getting on donkey (4)
I believe the answer is:
knit
'join' is the definition.
(knitting is a kind of joining)
'king getting on donkey' is the wordplay.
'king' becomes 'k'.
'getting on' means one lot of letters go next to another.
'donkey' becomes 'nit' (both can mean a stupid person).
'k'+'nit'='KNIT'
(Other definitions for knit that I've seen before include "Make woollen objects" , "Unite closely" , "Cause to unite" , "Fuse" , "Make a sweater, maybe" .)