Henri's explosive Eau de Cologne? (5)
I believe the answer is:
rhine
'eau de cologne?' is the definition.
(river in Cologne - 'eau' means 'water' in French)
'henri's explosive' is the wordplay.
'explosive' indicates an anagram.
'henri' is an anagram of 'RHINE'.
(Other definitions for rhine that I've seen before include "River of Cologne" , "fed by 8 [RHUR]" , "Cologne's river" , "valley" , "Flower" .)