Part of the seaweed served up in sample? Never again! (4)
I believe the answer is:
agar
'part of the seaweed' is the definition.
'agar' can be an answer for 'seaweed' (I've seen this before). I'm not sure about the 'part of the' bit.
'up in sample? never again' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates a hidden word.
'raga' is hidden in the letters of 'sample never again'.
'raga' backwards is 'AGAR'.
'served' is the link.
Can you help me to learn more?
(Other definitions for agar that I've seen before include "Seaweed jelly" , "found in the sea" , "part of culture?" , "Food thickening agent obtained from seaweed" , "contribution to culture?" .)