One gliding along, fishy pupil at Eton, say (12)
I believe the answer is:
skateboarder
'one gliding along' is the definition.
(someone gliding on a skateboard)
'fishy pupil at eton say' is the wordplay.
'fishy' becomes 'skate' (type of fish).
'pupil at eton say' becomes 'boarder' (Eton is a boarding school).
'skate'+'boarder'='SKATEBOARDER'
(Other definitions for skateboarder that I've seen before include "one may roll by" , "Someone on a short plank with wheels" , "sportsman" , "Person riding on a low platform fitted with wheels" , "One self-propelled on wheels" .)