Navy member, nothing without leader (6)
I believe the answer is:
'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'.
(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" .)