Vital to have seen scary criminal (9)
I believe the answer is:
necessary
'vital' is the definition.
(I've seen this before)
'seen scary criminal' is the wordplay.
'criminal' indicates anagramming the letters.
'seen'+'scary'='seenscary'
'seenscary' is an anagram of 'NECESSARY'.
'to have' is the link.
(Other definitions for necessary that I've seen before include "Inescapable" , "Indispensable, essential" , "Essential, needed" , "the money?" , "Requisite" .)