Snake taken initially on board American vessels (1-5)
I believe the answer is:
u-boats
'vessels' is the definition.
(U-boat is an example)
'snake taken initially on board american' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'initially' suggests taking the first letters.
'on board' indicates putting letters inside.
'american' becomes 'us' (abbreviation for United States).
The initial letter of 'taken' is 't'.
'boa'+'t'='boat'
'boat' going within 'us' is 'U-BOATS'.
(Other definitions for u-boats that I've seen before include "German submarines of second World War" , "German subs" , "Submersible ships of WW2" , "German submarines in WW2" , "Submersible warships" .)