Fleet applies 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' inserted inside 'raid' is 'RAPID'.
'applies' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rapid that I've seen before include "Electrical connector" , "Flying" , "moving fast" , "Waterway where the current runs very fast" , "Fast, quick" .)