Swimmer caught the flu -- it's bad (10)
I believe the answer is:
cuttlefish
'swimmer' is the definition.
(type of fish, something which swims)
'caught the flu it's bad' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'bad' indicates anagramming the letters.
'the'+'flu'+'its'='thefluits'
'thefluits' with letters rearranged gives 'uttlefish'.
'c'+'uttlefish'='CUTTLEFISH'
(Other definitions for cuttlefish that I've seen before include "Mollusc - leftish cut (anag)" , "An inshore cephalopod" , "Squid's cousin" , "Mollusc, source of sepia" , "Squid-like marine mollusc" .)