He joins us at golf (3)
I believe the answer is:
gus
'he joins' is the definition.
The definition suggests a singular noun which matches the answer.
'us at golf' is the wordplay.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'us' put after 'g' is 'GUS'.
Can you help me to learn more?
(Other definitions for gus that I've seen before include "T. S. Eliot's theatre cat" , "Augustus (dim.)" .)