What may be ridden round by one, briefly? (8)
I believe the answer is:
unicycle
'what may be ridden' is the definition.
'round by one briefly?' is the wordplay.
'round' becomes 'cycle' ('cycle' can be a synonym of 'round'**).
'by' is a charade indicator (letters next to each other).
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'briefly?' means to remove the last letter (I've seen 'briefly' mean this).
'unit' with its last letter removed is 'uni'.
'cycle' after 'uni' is 'UNICYCLE'.
(Other definitions for unicycle that I've seen before include "form of transport" , "Balancing act" , "One saddled" , "One-wheeled conveyance" , "One-wheeled bike used by acrobats" .)