Nuclear submarine I'd put in river (7)
I believe the answer is:
trident
'nuclear submarine' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'i'd put in river' is the wordplay.
'put in' indicates putting letters inside.
'river' becomes 'trent' (River Trent).
'id' placed into '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" .)