Poling a boat topless round back of Trinity? It needed this first (7)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

{p}UNTING (poling a boat) [topless] containing [round] {Trinit}Y [back of…]. A nice surface, as those familiar with Cambridge will know that Trinity is one of the colleges that backs on to the River Cam, a stretch that’s popular for punting in the city centre and referred to locally as ‘The Backs’.