Prepare for action keel scratched at sea (5,3,5)
I believe the answer is:
clear the decks
'prepare for action' is the definition.
(I've seen this before)
'keel scratched at sea' is the wordplay.
'at sea' indicates an anagram.
'keel'+'scratched'='keelscratched'
'keelscratched' anagrammed gives 'CLEAR THE DECKS'.
(Other definitions for clear the decks that I've seen before include "Tidy up all suits on hand to" , "Remove obstacles to action" , "Prepare for something by dealing first with matters outstanding" , "Get shipshape - prepare for action" , "Tidy up after card-games" .)