Fleet of foot, after short swim (5)
I believe the answer is:
swift
'fleet' is the definition.
('swift' can be a synonym of 'fleet')
'foot after short swim' is the wordplay.
'foot' becomes 'ft' ('ft' can be a synonym of 'foot').
'after' means one lot of letters go next to another.
'short' means to remove the last letter.
'swim' with its final letter removed is 'swi'.
'ft' put after 'swi' is 'SWIFT'.
'of' acts as a link.
(Other definitions for swift that I've seen before include "Quick - writer" , "Fast - novelist" , "Expeditious" , "wit?" , "Speedy - bird like a swallow" .)