Shortly summon agent about love song (7)
I believe the answer is:
'love song' is the definition.
'shortly summon agent about' is the wordplay.
I cannot really understand how this works, but
'about' could be 'ca' (abbreviation for 'circa') and 'ca' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for calypso that I've seen before include "sarcastic air" , "Hostage-holding nymph" , "Improvised song of the Caribbean" , "Ballad - dance - orchid - sea nymph - moon of Saturn" , "W.Indian ballad dealing with topical events" .)