Merry-go-round a man observed across river (8)
I believe the answer is:
carousel
'merry-go-round' is the definition.
(synonyms)
'a man observed across river' is the wordplay.
'a man' becomes 'carl' ().
'observed across' indicates putting letters inside.
'river' becomes 'ouse' (Yorkshire river).
'carl' enclosing 'ouse' is 'CAROUSEL'.
(Other definitions for carousel that I've seen before include "Does Carol use this piece of fairground equipment?" , "Revolving belt for luggage at airport" , "luggage mover at Heathrow?" , "A closure (anag.)" , "magazine for circulation" .)