Mouth of a brisk Yorkshire river (8)
I believe the answer is:
aperture
'mouth' is the definition.
(thesaurus)
'a brisk yorkshire river' is the wordplay.
'brisk' becomes 'pert'.
'yorkshire river' becomes 'ure' (I've seen this in other clues).
'a'+'pert'+'ure'='APERTURE'
'of' is the link.
(Other definitions for aperture that I've seen before include "An opening that admits light" , "Opening (for a photographer?)" , "Hole, gap" , "Opening through which light enters a camera" , "Opening (on camera, for example)" .)