One cutting through breakwater (4)
I believe the answer is:
pier
'breakwater' is the definition.
(I've seen this before)
'one cutting through' is the wordplay.
'one' becomes 'i' (Roman numeral).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'through' becomes 'per' (I've seen this before).
'i' put within 'per' is 'PIER'.
(Other definitions for pier that I've seen before include "Upright member" , "Support for two adjacent bridge spans" , "Mole, wall between windows" , "Ripe as a jetty (4)" , "Wharf jutting out into water" .)