American substitute porter, perhaps welcoming group of three visiting (9)
I believe the answer is:
alternate
'american substitute' is the definition.
'porter perhaps welcoming group of three visiting' is the wordplay.
'porter perhaps' becomes 'ale' (porter is a kind of ale).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'group of three' becomes 'tern' (rare term for a group of three).
'visiting' becomes 'at'.
'tern'+'at'='ternat'
'ale' enclosing 'ternat' is 'ALTERNATE'.
(Other definitions for alternate that I've seen before include "Change to first the one, then the other" , "deputy" , "Flip" , "Change from one thing to another by turns" , "Take turns" .)