A lot of taxis are fast (5)
I believe the answer is:
'are fast' is the definition.
(similar in meaning)
'a lot of taxis' is the wordplay.
'a lot' means to remove the first letter (I've seen 'a lot of' mean this).
'taxis' becomes 'leet' (I can't justify this - if you can you should give a lot more credence to this answer).
'of' with its initial letter taken off is 'f'.
'f'+'leet'='FLEET'
Can you help me to learn more?
(Other definitions for fleet that I've seen before include "group of taxis, for example" , "ships, lots of them" , "Swift; group of ships" , "Quick of foot" , "Fast-moving" .)