Locate it in the middle of Ossett (4)
I believe the answer is:
site
'locate' is the definition.
('site' can be a synonym of 'locate')
'it in the middle of ossett' is the wordplay.
'in the' means one lot of letters goes inside another.
'middle of' means to look at the middle letters.
The middle letters of 'ossett' are 'se'.
'it' going inside 'se' is 'SITE'.
(Other definitions for site that I've seen before include "Whereabouts" , "Ties into building location" , "Locality" , "Position; situation" , "Location (of building, dig etc)" .)