Cuttlefish very good in sea setting (5)
I believe the answer is:
sepia
'cuttlefish' is the definition.
(I've seen this before)
'in sea setting' is the wordplay.
'in' indicates putting letters inside.
'setting' becomes 'pi' (I can't justify this - if you can you should believe this answer much more).
'sea' enclosing 'pi' is 'SEPIA'.
'very good' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sepia that I've seen before include "Brown tone associated with old photographs" , "Sort of drawing" , "Brownish tint or pigment" , "brownish colour" , "Reddish-brown; cuttlefish ink" .)