Together or mainly by oneself -- golf (5)
I believe the answer is:
along
'together' is the definition.
(I've seen this in another clue)
'mainly by oneself golf' is the wordplay.
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'by oneself' becomes 'alone' (being by oneself is being alone).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'alone' with its final letter taken away is 'alon'.
'alon'+'g'='ALONG'
'or' is the link.
(Other definitions for along that I've seen before include "For the length of" , "Forward - in a line" , "Throughout; onward" , "Lining" , "From one end to the other" .)