It’s found in coal gas and seaweed (4)
I believe the answer is:
alga
'seaweed' is the definition.
(singular form of 'algae')
'it's found in coal gas and' is the wordplay.
'it's found in' indicates the answer is hidden within the clue.
'ALGA' is hidden within 'coal gas and'.
(Other definitions for alga that I've seen before include "Seaweed family group" , "Possibly tangle" , "Simple, non-flowering, aquatic plant" , "Simple plant, plankton or seaweed say" , "Eg, seaweed" .)