Layer of art must be modified (7)
I believe the answer is:
stratum
'layer' is the definition.
(I know that stratum is a type of layer)
'art must be modified' is the wordplay.
'be modified' indicates an anagram.
'art'+'must'='artmust'
'artmust' anagrammed gives 'STRATUM'.
'of' is the link.
(Other definitions for stratum that I've seen before include "Social class - level" , "Bed of sedimentary rock" , "Layer (of rock)" , "stretch of rock" , "rock band" .)