Extremely upset about snake, an unseen attacker (1-4)
I believe the answer is:
u-boat
'an unseen attacker' is the definition.
(German submarine in WW1 and WW2)
'extremely upset about snake' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'about' indicates putting letters inside.
'snake' becomes 'boa' (boa is a kind of snake).
'upset' with its middle removed is 'ut'.
'ut' enclosing 'boa' is 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "Take pool exercise" , "German World War One or World War Two submarine" , "German submarine of WW2" , "About (anag)" , "Sea vessel" .)