Rum initially reviving unfamiliar caller, perhaps (8)
I believe the answer is:
stranger
'unfamiliar caller perhaps' is the definition.
(unfamiliar visitor)
'rum initially reviving' is the wordplay.
'rum' becomes 'strange' (similar in meaning).
'initially reviving' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'strange'+'r'='STRANGER'
(Other definitions for stranger that I've seen before include "Outsider" , "More exotic" , "Unfamiliar person" , "Odder, more peculiar" , "Alien, foreigner" .)