Seaweed found among the coastal garbage (4)
I believe the answer is:
alga
'seaweed' is the definition.
(singular form of 'algae')
'among the coastal garbage' is the wordplay.
'among' indicates a hidden word.
'ALGA' is hidden in the letters of 'coastal garbage'.
'found' is the link.
(Other definitions for alga that I've seen before include "Primitive non-flowering, mainly aquatic, plant" , "Plants such as seaweed" , "Gala (anag.)" , "Eg, seaweed" , "Irish moss, for example" .)