Art must work in layer (7)
I believe the answer is:
stratum
'layer' is the definition.
(I know that layer can be written as stratum)
'art must work' is the wordplay.
'work' is an anagram indicator.
'art'+'must'='artmust'
'artmust' is an anagram of 'STRATUM'.
'in' acts as a link.
(Other definitions for stratum that I've seen before include "rock band" , "Layer (of rock)" , "Social class - level" , "Bed of sedimentary rock" , "stretch of rock" .)