Snake appearing in film — through water it sped (1-4)
I believe the answer is:
e-boat
'through water it sped' is the definition.
'snake appearing in film' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'appearing in' is an insertion indicator.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'boa' placed within 'et' is 'E-BOAT'.
(Other definitions for e-boat that I've seen before include "German WW2 torpedo vessel" , "Enemy vessel" , "Torpedo-vessel" , "German WWII fast motor torpedo vessel" .)