Brief suggestion by retired police officer for swimmer (7)
I believe the answer is:
dolphin
'swimmer' is the definition.
(dolphin is a kind of aquatic vertebrate)
'brief suggestion by retired police officer' is the wordplay.
'brief suggestion' becomes 'hin' (I am not sure about this - if you are sure you should believe this answer much more).
'by' says to put letters next to each other.
'retired' is a reversal indicator.
'police officer' becomes 'plod' (plod can informally mean a police officer).
'plod' back-to-front is 'dolp'.
'hin' put after 'dolp' is 'DOLPHIN'.
'for' is the link.
Can you help me to learn more?
(Other definitions for dolphin that I've seen before include "Small sociable whale" , "Small toothed whale, larger than a porpoise" , "Sea-mammal" , "Small (bottlenosed) whale" , "Relative of whale" .)