To start riding something very high is dodgy (5)
I believe the answer is:
risky
'dodgy' is the definition.
(synonyms)
'to start riding something very high is' is the wordplay.
I cannot quite see how this works, but
'is' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for risky that I've seen before include "admitting possibility of unpleasant outcome" , "Having possibility of danger or loss" , "Involving a chance of danger" , "not playing safe" , "Perilous" .)