Nuclear submarine I'd put in river (7)
I believe the answer is:
trident
'nuclear submarine' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'i'd put in river' is the wordplay.
'put in' means one lot of letters goes inside another.
'river' becomes 'trent' (River Trent).
'id' placed inside 'trent' is 'TRIDENT'.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "Fork perhaps" , "Three-pronged weapon" , "Nuclear missile" , "Neptune's spear - missile" , "not pike" .)