Nuclear submarine I'd put in river (7)
'nuclear submarine' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps 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' (I've seen this before).
'id' inserted within '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".)