Main pressure contains opening of main sewer (10)
I believe the answer is:
seamstress
'sewer' is the definition.
(thesaurus)
'main pressure contains opening of main' is the wordplay.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'pressure' becomes 'stress' (I've seen this before**).
'contains' means one lot of letters goes inside another.
'opening of' says to take the initial letters.
The initial letter of 'main' is 'm'.
'sea'+'stress'='seastress'
'seastress' placed around 'm' is 'SEAMSTRESS'.
(Other definitions for seamstress that I've seen before include "needlewoman" , "Woman who sews for a living" , "Dressmaker" , "sewer" , "Midinette" .)