Swear Madagascar is concealing fleet (6)
I believe the answer is:
armada
'fleet' is the definition.
(armada is a kind of fleet)
'swear madagascar is concealing' is the wordplay.
'is concealing' indicates a hidden word.
'ARMADA' is hidden within 'swear madagascar'.
(Other definitions for armada that I've seen before include "Large flotilla" , "Large Spanish fleet" , "Navy" , "Spanish war fleet" , "Large number of ships (Spanish?)" .)