Navy (5)
'navy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fleet that I've seen before include "collection of aircraft" , "Quick of foot" , "Group of vehicles operating together, ships say" , "Swift; group of ships" , "Swift like naval force" .)
(Other definitions for seals that I've seen before include "Fastens or closes securely" , "they're impressive" , "Makes waterproof" , "They make an impression" , "Marine mammals" .)