Tremendous macaroni -- lots to be used (12)
I believe the answer is:
astronomical
'tremendous' is the definition.
(I've seen this before)
'macaroni lots to be used' is the wordplay.
'to be used' is an anagram indicator.
'macaroni'+'lots'='macaronilots'
'macaronilots' is an anagram of 'ASTRONOMICAL'.
(Other definitions for astronomical that I've seen before include "Tremendous" , "Enormous" , "Massive" , "Of the study of stars" , "Absolutely huge, in a heavenly way" .)