Suspicious swimmer by end of quay (5)
I believe the answer is:
fishy
'suspicious' is the definition.
(synonyms)
'swimmer by end of quay' is the wordplay.
'swimmer' becomes 'fish' (fish swim).
'by' means one lot of letters go next to another.
'end of' says to take the final letters.
The final letter of 'quay' is 'y'.
'fish'+'y'='FISHY'
(Other definitions for fishy that I've seen before include "As smelt, perhaps" , "Dodgy, suspicious" , "Arousing suspicion" , "Suspicious (as trout?)" , "Ray is so" .)