Pots are made of grass (7)
I believe the answer is:
esparto
'grass' is the definition.
(type of grass plant)
'pots are made' is the wordplay.
'made' indicates anagramming the letters.
'pots'+'are'='potsare'
'potsare' is an anagram of 'ESPARTO'.
'of' acts as a link.
(Other definitions for esparto that I've seen before include "Fibrous grass" , "Coarse type of grass" , "Grass fibre for mat-making" , "A poster (anag.)" , "Source of paper" .)