One's scrutinised an explosive device in river (8)
I believe the answer is:
examinee
'one's scrutinised' is the definition.
(someone being examined or scrutinised)
'an explosive device in river' is the wordplay.
'an' becomes 'a'.
'explosive device' becomes 'mine' (mine is a kind of explosive device).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'a'+'mine'='amine'
'amine' placed within 'exe' is 'EXAMINEE'.
(Other definitions for examinee that I've seen before include "One taking formal test" , "Test taker" , "one facing Test" , "One's tested" , "One being tested" .)