French underground force formed by quasi-military leader (6)
I believe the answer is:
maquis
'french underground force' is the definition.
(French resistance group in WW2)
'formed by quasi-military leader' is the wordplay.
'formed' indicates an anagram.
'by' says to put letters next to each other.
'leader' suggests taking the first letters.
The first letter of 'military' is 'm'.
'quasi' put after 'm' is 'mquasi'.
'mquasi' anagrammed gives 'MAQUIS'.
(Other definitions for maquis that I've seen before include "French underground force" , "underground movement" , "Frenchman fighting" , "dense scrubby vegetation around the Med" , "World War Two French resistance" .)