One can ride in it, as a rule (5)
I believe the answer is:
habit
'a rule' is the definition.
(thesaurus)
'one can ride in it' is the wordplay.
'one can ride in' becomes 'hab' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hab'+'it'='HABIT'
'as' is the link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "can cause addiction" , "Settled practice" , "Propensity" , "Settled tendency" , "nun gets into" .)