Familiar with riding? (4)
I believe the answer is:
upon
'familiar with riding?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'familiar with riding?' is the wordplay.
'familiar with' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'riding?' becomes 'on' (I've seen this in other clues).
'up'+'on'='UPON'
Can you help me to learn more?
(Other definitions for upon that I've seen before include "Preposition" , "mounted" , "voucher, business stopped" , "Supported by (a surface)" , "Thereafter" .)