A hundred take it easy on the ridge (5)
I believe the answer is:
crest
'the ridge' is the definition.
(I've seen this before)
'a hundred take it easy on' is the wordplay.
'a hundred' becomes 'C' (Roman numerals).
'take it easy' becomes 'rest' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'c'+'rest'='CREST'
(Other definitions for crest that I've seen before include "Wave top; (bird's) comb" , "Highest point" , "Summit ridge" , "Summit - emblem - cockscomb" , "Peak - comb" .)