A lot of Americans fish on a boat (3,4)
I believe the answer is:
car park
'a lot of americans' is the definition.
(Americans call a car park a parking lot)
'fish on a boat' is the wordplay.
'fish' becomes 'carp' (carp is a kind of fish).
'on' means one lot of letters go next to another.
'a boat' becomes 'ark' (ark is a kind of boat).
'carp'+'ark'='CAR PARK'
(Other definitions for car park that I've seen before include "multistorey, for example - with 27d" , "of which many stories might be told" , "Where vehicles are left" , "Place to leave vehicles" , "Multi-storey building with bays" .)