Seaweed from a coastal garden (4)
I believe the answer is:
alga
'seaweed' is the definition.
(singular form of 'algae')
'from a coastal garden' is the wordplay.
'from' indicates a hidden word.
'ALGA' is hidden within 'coastal garden'.
(Other definitions for alga that I've seen before include "Aquatic organism such as plankton" , "Autotrophic organism" , "Irish moss, for example" , "Primitive non-flowering, mainly aquatic, plant" , "Gala (anag.)" .)