Bear ought to explore river sources (8)
I believe the answer is:
shoulder
'bear' is the definition.
(thesaurus)
'ought to explore river sources' is the wordplay.
'ought to' becomes 'should' (I've seen this before).
'sources' says to take the initial letters.
The first letters of 'explore river' is 'er'.
'should'+'er'='SHOULDER'
(Other definitions for shoulder that I've seen before include "Meat joint" , "Undertake (burden)" , "Bear < to push" , "take responsibility?" , "On the side of the road, it might be hard" .)