Swimmer flagging at the outset, kind of (4)
I believe the answer is:
fish
'swimmer' is the definition.
(fish swim)
'flagging at the outset kind of' is the wordplay.
'at the outset' says to take the initial letters (the outset is the beginning).
'kind of' becomes 'ish' (suffix meaning 'kind of').
The first letter of 'flagging' is 'f'.
'f'+'ish'='FISH'
(Other definitions for fish that I've seen before include "Ruff" , "Weatherman" , "Swimmer" , "Trawler's catch" , "Aquatic animal" .)