He joins us at golf (3)
I believe the answer is:
gus
'he' is the definition.
'us at golf' is the wordplay.
'at' means one lot of letters go next to another (I've seen this in other clues).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'us' put after 'g' is 'GUS'.
'joins' is the link.
(Other definitions for gus that I've seen before include "T. S. Eliot's theatre cat" , "Augustus (dim.)" .)