Snake in dock caught leaving submarine (1-4)
I believe the answer is:
u-boat
'submarine' is the definition.
(I've seen this before)
'snake in dock caught leaving' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'in' is an insertion indicator.
'dock' becomes 'cut'.
'caught' becomes 'c' (cricket abbreviation).
'leaving' is a deletion indicator.
'cut' with 'c' removed is 'ut'.
'boa' placed within 'ut' is 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "Submersible warship armed with torpedoes" , "German wartime submarine" , "German World War One or World War Two submarine" , "Sea vessel" .)