Fast navy (5)
I believe the answer is:
fleet
This clue is a double definition.
'fast' is the first definition.
(similar in meaning)
'navy' is the second definition.
(I've seen this before)
(Other definitions for fleet that I've seen before include "Navy - quick" , "Fast - naval ships" , "good turn of speed" , "Rapid in movement" , "Fast - group of boats" .)