Cliff is no end frightening (4)
I believe the answer is:
scar
'cliff' is the definition.
(I've seen this before)
'no end frightening' is the wordplay.
'no end' means to remove the last letter.
'frightening' becomes 'scary' (I've seen this in another clue).
'scary' with its final letter removed is 'SCAR'.
'is' is the link.
(Other definitions for scar that I've seen before include "area of wound" , "reef" , "Disfigure" , "sign of damage" , "Evidence of a past injury?" .)