Fleet Street gathering with a condition (5)
I believe the answer is:
swift
'fleet' is the definition.
(I know that fleet can be written as swift)
'street gathering with a condition' is the wordplay.
'street' becomes 'st' (abbreviation).
'gathering' indicates putting letters inside (gather can mean to acquire or bring in).
'with' becomes 'w' (short for with).
'a condition' becomes 'if' (as in 'a big if').
'w'+'if'='wif'
'st' enclosing 'wif' is 'SWIFT'.
(Other definitions for swift that I've seen before include "Quick, prompt" , "wit?" , "Fast-moving bird" , "Expeditious" , "Fast; bird; writer" .)