Group of three, heading off for Brazilian port (3)
I believe the answer is:
rio
'brazilian port' is the definition.
(I've seen this before)
'group of three heading off' is the wordplay.
'group of three' becomes 'trio' (I've seen this before).
'heading off' means to remove the first letter.
'trio' with its initial letter removed is 'RIO'.
'for' acts as a link.
(Other definitions for rio that I've seen before include "A Mexican river, may be grande" , "Flying Down to -, Astaire, Rogers film" , "Brazilian city and port" , "Location of next Olympic Games, in short" , "Port, -- de Janeiro" .)