Destroy European resistance on choppy sea (5)
I believe the answer is:
erase
'destroy' is the definition.
(thesaurus)
'european resistance on choppy sea' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'resistance' becomes 'r' (abbreviation).
'on' indicates putting letters inside.
'choppy' indicates anagramming the letters.
'sea' anagrammed gives 'ase'.
'r' put into 'ase' is 'rase'.
'e'+'rase'='ERASE'
(Other definitions for erase that I've seen before include "Eradicate" , "Clear" , "Remove permanently" , "Delete" , "Rub out, cancel" .)