Suspicious swimmer beginning to yield (5)
I believe the answer is:
fishy
'suspicious' is the definition.
(synonyms)
'swimmer beginning to yield' is the wordplay.
'swimmer' becomes 'fish' (fish swim).
'beginning to' says to take the initial letters.
The initial letter of 'yield' is 'y'.
'fish'+'y'='FISHY'
(Other definitions for fishy that I've seen before include "Dodgy, suspicious" , "Suspicious in piscine way" , "Suspicious (as trout?)" , "As smelt, perhaps" , "Questionable" .)