A great looming shifting mass (13)
I believe the answer is:
agglomeration
'mass' is the definition.
(thesaurus)
'a great looming shifting' is the wordplay.
'shifting' is an anagram indicator.
'great'+'looming'='greatlooming'
'greatlooming' with letters rearranged gives 'gglomeration'.
'a'+'gglomeration'='AGGLOMERATION'
(Other definitions for agglomeration that I've seen before include "assemblage" , "Clump" , "Great Mongolia (anag) - mass or collection of things" .)