Note circling reptile is a threat, in the main (1-4)
I believe the answer is:
u-boat
'threat in the main' 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?
'note circling reptile' is the wordplay.
'note' becomes 'ut' (I've seen this in other clues**).
'circling' is an insertion indicator (circling can mean going around or containing).
'reptile' becomes 'boa' (boa is a kind of reptile).
'ut' enclosing 'boa' is 'U-BOAT'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for u-boat that I've seen before include "German wartime submarine" , "Submersible warship armed with torpedoes" , "German WW 2 sub" , "Submersible warship of WWII" , "Take pool exercise" .)