Flood in a French department at the end of June (8)
I believe the answer is:
inundate
'flood' is the definition.
(I know that flood can be written as inundate)
'in a french department at the end of june' is the wordplay.
'a french' becomes 'un' ('a' in French).
'department' becomes 'd' (abbreviation eg in MOD Ministry of Defence).
'the end of' suggests the final letters.
The last letter of 'june' is 'e'.
'in'+'un'+'d'+'at'+'e'='INUNDATE'
(Other definitions for inundate that I've seen before include "Flood, cover with water" , "'Flood, swamp (8)'" , "Aunt Enid could engulf badly" , "Flood, fill completely" , "be overwhelming" .)