Impetuous Dean, discovered nude entering Scottish river (8)
I believe the answer is:
tearaway
'impetuous' is the definition.
(I know that impetuous can be written as tearaway)
'dean discovered nude entering scottish river' is the wordplay.
'discovered' indicates the central letters (dis-covered - outer letters removed).
'nude' becomes 'raw' ().
'entering' indicates putting letters inside.
'scottish river' becomes 'tay' (the River Tay in Scotland).
The centre of 'dean' is 'ea'.
'ea'+'raw'='earaw'
'earaw' placed inside 'tay' is 'TEARAWAY'.
(Other definitions for tearaway that I've seen before include "Yob" , "Hothead" , "Unruly youth" , "A tough" , "Reckless violent person" .)