Rigid seaweed is pulled up (4)
I believe the answer is:
iron
'rigid' is the definition.
(thesaurus)
'seaweed is pulled up' is the wordplay.
'seaweed' becomes 'nori' (I've seen this before).
'is pulled up' says the letters should be written backwards.
'nori' back-to-front is 'IRON'.
(Other definitions for iron that I've seen before include "Metallic chemical element" , "rations in emergency?" , "See 14" , "Main part of steel" , "Smooth things" .)