Mostly upriver, uncommonly wide range (7)
I believe the answer is:
purview
'range' is the definition.
(I've seen this before)
'mostly upriver uncommonly wide' is the wordplay.
'mostly' means to remove the last letter.
'uncommonly' is an anagram indicator.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'upriver' with its final letter taken away is 'uprive'.
'uprive' anagrammed gives 'purvie'.
'purvie'+'w'='PURVIEW'
(Other definitions for purview that I've seen before include "Range of thought" , "Scope of concern" , "Scope, range" , "Range (of experience/thought)" , "... experience range" .)