Suspicious swimmer by end of quay (5)
I believe the answer is:
fishy
'suspicious' is the definition.
(I know that suspicious can be written as fishy)
'swimmer by end of quay' is the wordplay.
'swimmer' becomes 'fish' (fish swim).
'by' is a charade indicator (letters next to each other).
'end of' suggests the final letters.
The last 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" .)