Discover a French planet (7)
I believe the answer is:
unearth
'discover' is the definition.
(to unearth is to discover or find)
'a french planet' is the wordplay.
'a french' becomes 'un' ('a' in French).
'planet' becomes 'earth' (Earth is an example).
'un'+'earth'='UNEARTH'
(Other definitions for unearth that I've seen before include "Dig up or find for haunter" , "Dig up for a hunter" , "Track down" , "Exhume" , "Expose" .)