Fleet applying pressure in attack (5)
I believe the answer is:
rapid
'fleet' is the definition.
(I've seen this before)
'pressure in attack' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'in' means one lot of letters goes inside another.
'attack' becomes 'raid' (raid is a kind of attack).
'p' put within 'raid' is 'RAPID'.
'applying' acts as a link.
(Other definitions for rapid that I've seen before include "Speedy, quick" , "Happening at great speed" , "Waterway where the current runs very fast" , "moving fast" , "Quick or swift" .)