Fast naval force (5)
I believe the answer is:
fleet
This clue is a double definition.
'fast' is the first definition.
('fleet' can be similar in meaning to 'fast')
'naval force' is the second definition.
(I've seen this before)
(Other definitions for fleet that I've seen before include "Fast moving" , "Fast and nimble" , "Fast as a group of ships" , "Collection of company cars" , "ships, lots of them" .)