Crack troops guarding European waters (4)
I believe the answer is:
seas
'waters' is the definition.
(sea is a kind of water)
'crack troops guarding european' is the wordplay.
'crack troops' becomes 'SAS' (Special Air Service).
'guarding' means one lot of letters goes inside another (inserted letters are protected or guarded).
'european' becomes 'E' (abbreviation e.g. EU).
'sas' placed around 'e' is 'SEAS'.
(Other definitions for seas that I've seen before include "Aegean and Mediterranean, say" , "Oceans" , "Red and Dead" , "Red and Black are" , "Bodies of salt water" .)