A lot of excellent aluminium is essential (6)
I believe the answer is:
primal
'essential' is the definition.
(thesaurus)
'a lot of excellent aluminium' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'excellent' becomes 'prime'.
'aluminium' becomes 'al' ('Al' can be a synonym of 'aluminium').
'prime' with its final letter removed is 'prim'.
'prim'+'al'='PRIMAL'
'is' acts as a link.
(Other definitions for primal that I've seen before include "Original - basic" , "Original, fundamental" , "Having existed from the beginning" , "Existing from the beginning, basic" , "Original or primitive" .)