A boat going under a bridge (7)
I believe the answer is:
pontoon
'a boat' is the definition.
(pontoon is a kind of boat)
'going under a bridge' is the wordplay.
I cannot quite understand how this works, but
'going' could be 'on' (both can mean functioning or activated) and 'on' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pontoon that I've seen before include "'Flat-bottomed boat, card game and Mayo town (7)'" , "Card game; float" , "Gambling game" , "Vingt-et-un" , "Card-game" .)