Study about river, backing required (6)
I believe the answer is:
needed
'required' is the definition.
('need' can be a synonym of 'require')
'study about river backing' is the wordplay.
'study' becomes 'den' (I've seen this before).
'about' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'backing' shows that the letters should be reversed in order.
'den' going around 'dee' is 'dedeen'.
'dedeen' reversed gives 'NEEDED'.
(Other definitions for needed that I've seen before include "Essential; called for" , "Was in want of" , "Required; sounds like worked (dough)" , "Required, necessary" , "'Lacked, required (6)'" .)