Steep cliff is mostly frightening (4)
I believe the answer is:
scar
'steep cliff' is the definition.
(scar can mean a large cliff)
'mostly frightening' is the wordplay.
'mostly' means to remove the last letter.
'frightening' becomes 'scary' (I've seen this before).
'scary' with its last letter taken away is 'SCAR'.
'is' is the link.
(Other definitions for scar that I've seen before include "reef" , "Limestone cliff or outcrop" , "Old wound - bare cliff" , "Permanently damage" , "Disfigure" .)