Dictator engaged in river survey (7)
I believe the answer is:
examine
'survey' is the definition.
(I know that survey is a more specific form of examine)
'dictator engaged in river' is the wordplay.
'dictator' becomes 'amin' (Idi Amin, Ugandan dictator).
'engaged in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'amin' inserted inside 'exe' is 'EXAMINE'.
(Other definitions for examine that I've seen before include "Subject to scrutiny" , "Subject to inspection" , "Look closely at < test" , "research" , "Question, test" .)