Ploy devised by quiet sea creature (5)
I believe the answer is:
polyp
'sea creature' is the definition.
'polyp' can be an answer for 'creature' (polyp is a kind of creature). I'm not certain of the 'sea' bit.
'ploy devised by quiet' is the wordplay.
'devised' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'ploy' with letters rearranged gives 'poly'.
'poly'+'p'='POLYP'
Can you help me to learn more?
(Other definitions for polyp that I've seen before include "Small growth, usually benign" , "Small tumour" , "Small growth (medical)" , "Coral form" , "Small growth on a mucous membrane" .)