Fortunate I am on horse: no fun here on bike? (12)
I believe the answer is:
cobblestones
'no fun here on bike?' is the definition.
(a bike would bump over cobblestones)
'fortunate i am on horse' is the wordplay.
'fortunate' becomes 'blest' (archaic form of blessed).
'i am' becomes 'one's' ('one's' or 'one is' can humorously mean 'I am').
'on' means one lot of letters go next to another.
'horse' becomes 'cob' (type of horse).
'blest'+'ones'='blestones'
'blestones' put after 'cob' is 'COBBLESTONES'.
(Another definition for cobblestones that I've seen is " They made way".)