Masses of salt water around motorway -- lock sewer (10)
I believe the answer is:
seamstress
'sewer' is the definition.
(someone who sews)
'masses of salt water around motorway lock' is the wordplay.
'masses of salt water' becomes 'seas' (seas are made up of salt water).
'around' indicates putting letters inside.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'lock' becomes 'tress' (both can mean a piece of hair).
'seas' placed around 'm' is 'seams'.
'seams'+'tress'='SEAMSTRESS'
(Other definitions for seamstress that I've seen before include "Midinette" , "Dressmaker" , "Woman who sews for a living" , "needlewoman" , "sewer" .)