Cloud’s position covering river (7)
I believe the answer is:
stratus
'cloud's' is the definition.
(stratus cloud is a kind of cloud)
'position covering river' is the wordplay.
'position' becomes 'status' (one's status or standing).
'covering' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'status' placed around 'r' is 'STRATUS'.
(Other definitions for stratus that I've seen before include "Large dark low cloud" , "Source of rain" , "Cloud as a grey sheet" , "Continuous cloud type" , "Sheet of low cloud" .)