Navy member, nothing without leader (6)
I believe the answer is:
armada
'navy' is the definition.
(I've seen this before)
'member nothing without leader' is the wordplay.
'member' becomes 'arm' (arm is a kind of member).
'nothing' becomes 'nada' ('nada' can be a synonym of 'nothing').
'without leader' means to remove the first letter.
'nada' with its first letter removed is 'ada'.
'arm'+'ada'='ARMADA'
(Other definitions for armada that I've seen before include "Large number of ships (Spanish?)" , "A great fleet - Spain had one" , "The Spanish force" , "Naval fighting force" , "Spanish war fleet" .)