Cliff -- mostly frightening (4)
I believe the answer is:
scar
'cliff' is the definition.
(I've seen this before)
'mostly frightening' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'frightening' becomes 'scary' (I've seen this before).
'scary' with its final letter taken away is 'SCAR'.
(Other definitions for scar that I've seen before include "Mark on body or tree" , "Leave wound mark" , "sign of damage" , "Mark left on healed tissue" , "A bare precipice" .)