Snake in posh tango the main threat? (1-4)
I believe the answer is:
u-boat
'main threat?' is the definition.
(main can mean open sea)
'snake in posh tango' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'in' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'u'+'t'='ut'
'boa' going into 'ut' is 'U-BOAT'.
'the' acts as a link.
(Other definitions for u-boat that I've seen before include "Submersible warship armed with torpedoes" , "German wartime submarine" , "German World War One or World War Two submarine" , "Take pool exercise" , "Submersible warship of WWII" .)