Grating, one obstructing harbour clearances (10)
I believe the answer is:
portcullis
'grating' is the definition.
(I've seen this before)
'one obstructing harbour clearances' is the wordplay.
'one' becomes 'i' (Roman numeral).
'obstructing' indicates putting letters inside (inserted letters interrupt or obstruct the word).
'harbour' becomes 'port' (I've seen this before).
'clearances' becomes 'culls' (a cull clears animals from an area).
'port'+'culls'='portculls'
'i' inserted inside 'portculls' is 'PORTCULLIS'.
(Other definitions for portcullis that I've seen before include "feature of castle" , "Castle gate" , "Sliding grating in castle" , "Grating at castle entrance" , "Seen from 13 [DRAWBRIDGE], perhaps" .)