Insufficient to frighten many into (6)
I believe the answer is:
scarce
'insufficient' is the definition.
('scarce' can be similar in meaning to 'insufficient')
'frighten many into' is the wordplay.
'frighten' becomes 'scare' ('scare' can be a synonym of 'frighten').
'many' becomes 'c' (C is the Roman numeral for 100).
'into' indicates putting letters inside.
'scare' enclosing 'c' is 'SCARCE'.
'to' is the link.
(Other definitions for scarce that I've seen before include "Insufficient to meet demand" , "exceptional" , "Hard to obtain" , "Like gold dust" , "Common? Definitely not" .)