Person who’s tested an explosive device in river (8)
I believe the answer is:
examinee
'person who's tested' is the definition.
(I've seen this before)
'an explosive device in river' is the wordplay.
'an' becomes 'a'.
'explosive device' becomes 'mine' (mine is a kind of explosive device).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'a'+'mine'='amine'
'amine' put into 'exe' is 'EXAMINEE'.
(Other definitions for examinee that I've seen before include "One being tested" , "Whipped dessert" , "One sitting a test" , "in France, one taking the Baccalaureat?" , "GCSE entrant" .)