Seaweed in the National Gallery (4)
I believe the answer is:
alga
'seaweed' is the definition.
(singular form of 'algae')
'in the national gallery' is the wordplay.
'in' indicates a hidden word.
'ALGA' can be found hidden inside 'national gallery'.
(Other definitions for alga that I've seen before include "Autotrophic organism" , "Gala (anag.)" , "Gala class of seaweed" , "nuisance on the beach?" , "from ore found locally" .)