Some in Gibraltar mad about fleet (6)
I believe the answer is:
armada
'fleet' is the definition.
(armada is a kind of fleet)
'some in gibraltar mad about' is the wordplay.
'some in' says the answer is hidden in the clue.
'ARMADA' can be found hidden inside 'gibraltar mad about'.
(Other definitions for armada that I've seen before include "vessels" , "1588 sea battle" , "Many ships" , "The Spanish force" , "Spanish war fleet" .)