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