Substitute for topsy-turvy exercise in river (8)
I believe the answer is:
deputise
'substitute for' is the definition.
('substitute for'->'deputize' is in my internal thesaurus and 'ize'->'ise')
'topsy-turvy exercise in river' is the wordplay.
'topsy turvy' says the letters should be written backwards.
'exercise' becomes 'situp' (I have seen 'Physical exercise ' mean 'sit-up' so perhaps 'exercise' could also mean 'sit-up').
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'situp' back-to-front is 'putis'.
'putis' inserted within 'dee' is 'DEPUTISE'.
(Other definitions for deputise that I've seen before include "Appoint as stand-in" , "Understudy" , "Appoint as representative" , "Act as delegate for a person" , "Stand in for" .)