Characters in sea garnered seaweed (4)
I believe the answer is:
agar
'seaweed' is the definition.
(I've seen this before)
'characters in sea garnered' is the wordplay.
'characters in' says the answer is hidden in the clue.
'AGAR' is hidden in the letters of 'sea garnered'.
(Other definitions for agar that I've seen before include "part of culture?" , "that aids culture" , "Jelly medium" , "found in the sea" , "Jelly used in bacteria culture" .)