Danger at sea: English prepare to fight! (2,5)
I believe the answer is:
en garde
'prepare to fight' is the definition.
(phrase said when fencing)
'danger at sea english' is the wordplay.
'at sea' indicates an anagram.
'english' becomes 'E' (abbreviation as in OED).
'danger' is an anagram of 'engard'.
'engard'+'e'='EN GARDE'
(Other definitions for en garde that I've seen before include "French opening position in fencing - protect yourself" , "Prepare to fence!" , "Fencer's warning cry" , "warning of attack" , "Words of warning" .)