Find a French soil (7)
I believe the answer is:
Here is my best explanation:
'find' is the definition.
(unearthing is a kind of finding)
'a french soil' is the subsidiary indication.
'a french' becomes 'un' ('a' transated into the French language).
'soil' becomes 'earth' (I've seen this before**).
(Other definitions for unearth that I've seen before include "Track down", "reveal", "Excavate, dig up", "Dig up or find for haunter", "Disinter", "Bring out of obscurity", "Expose", "dredge up", "Discover" and "Find by digging".)