Prow of icebreaker blocking a dock (4)
I believe the answer is:
pier
'dock' is the definition.
(I've seen this before)
'prow of icebreaker blocking a' is the wordplay.
'prow of' says to take the initial letters (prow means the front of a boat).
'blocking' indicates putting letters inside (to block can mean to plug a gap in something).
'a' becomes 'per' (eg 'one a day' means 'one per day').
The first letter of 'icebreaker' is 'i'.
'i' inserted inside 'per' is 'PIER'.
(Other definitions for pier that I've seen before include "Landing-stage or promenade" , "'Dock, wharf (4)'" , "Coastal platform" , "Wharf jutting out into water" , "Break-water or landing stage" .)