For excavation, good book is important (9)
I believe the answer is:
prominent
'important' is the definition.
(I've seen this before)
'for excavation good book' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'excavation' becomes 'mine' (mine is a kind of excavation).
'good book' becomes 'NT' (New Testament).
'pro'+'mine'+'nt'='PROMINENT'
'is' is the link.
(Other definitions for prominent that I've seen before include "Well-known" , "Conspicuous or outstanding" , "Projecting; famous" , "sticking up" , "Noticeable or notable" .)