Ugandan's leader on river (3)
I believe the answer is:
ure
'river' is the definition.
(river in Yorkshire)
'ugandan's leader on' is the wordplay.
'leader' suggests taking the first letters.
'on' becomes 're' (regarding).
The initial letter of 'ugandan' is 'u'.
'u'+'re'='URE'
(Other definitions for ure that I've seen before include "Ripon's river" , "North Yorkshire river" , "Midge --, singer" , "Wensleydale river" , "River at Ripon" .)