Swimmer ready to keep fit with training (7)
I believe the answer is:
catfish
'swimmer' is the definition.
(catfish is a type of fish - fish swim)
'ready to keep fit with training' is the wordplay.
'ready' becomes 'cash' (ready can informally mean cash).
'to keep' means one lot of letters goes inside another.
'with training' indicates an anagram (to train a plant is to manipulate its growth).
'fit' is an anagram of 'tfi'.
'cash' enclosing 'tfi' is 'CATFISH'.
(Other definitions for catfish that I've seen before include "Freshwater type" , "His fact (anag)" , "sea creature" , "Aquatic creature with barbels" , "Bewhiskered swimmer?" .)