Uncommon enough to frighten about 100 (6)
I believe the answer is:
scarce
'uncommon' is the definition.
(scarce means rare or uncommon)
'frighten about 100' is the wordplay.
'frighten' becomes 'scare' (synonyms).
'about' indicates putting letters inside.
's00' becomes 'C' (Roman numeral).
'scare' enclosing 'c' is 'SCARCE'.
'enough to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for scarce that I've seen before include "exceptional" , "Rarely found" , "Not often come across" , "Scant, meagre" , "Hardly" .)