Art Elgin shifted, a pediment perhaps (8)
I believe the answer is:
triangle
'a pediment perhaps' is the definition.
(pediment is a triangular architectural feature)
'art elgin shifted' is the wordplay.
'shifted' is an anagram indicator.
'art'+'elgin'='artelgin'
'artelgin' with letters rearranged gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Percussion instrument of geometric origin" , "kitchen item" , "complex romantic situation?" , "Three-sided shape" , "Simple percussion instrument" .)