Big city about to host golf (5)
I believe the answer is:
large
'big' is the definition.
(synonyms)
'city about to host golf' is the wordplay.
'city' becomes 'LA' (Los Angeles).
'about' becomes 're' (regarding).
'to host' means one lot of letters goes inside another (inserted letters are hosted or welcomed).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
're' enclosing 'g' is 'rge'.
'la'+'rge'='LARGE'
(Other definitions for large that I've seen before include "Immense" , "Lake" , "substantial" , "Little's late partner" , "Copious" .)