Important for pit not to be empty (9)
I believe the answer is:
prominent
'important' is the definition.
(I've seen this before)
'for pit not to be empty' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'pit' becomes 'mine' (I've seen this before).
'to be empty' says to hollow out the word (remove centre letters).
'not' with its middle removed is 'nt'.
'pro'+'mine'+'nt'='PROMINENT'
(Other definitions for prominent that I've seen before include "sticking up" , "Standing out" , "Conspicuous or outstanding" , "Projecting; famous" , "Well-known" .)