Regardless of small mine in river (7)
I believe the answer is:
despite
'regardless of' is the definition.
(I've seen this before)
'small mine in river' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mine' becomes 'pit' (down the pit).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
's'+'pit'='spit'
'spit' placed within 'dee' is 'DESPITE'.
(Other definitions for despite that I've seen before include "Undeterred" , "In the face" , "Without being affected by; contempt" , "Regardless of, in defiance of" , "Notwithstanding" .)