Fearless girl, leader in foreign incursion (8)
I believe the answer is:
unafraid
'fearless' is the definition.
(I know that fearless can be written as unafraid)
'girl leader in foreign incursion' is the wordplay.
'girl' becomes 'Una' (girl's name).
'leader in' suggests taking the first letters.
'incursion' becomes 'raid' (raid is a kind of incursion).
The first letter of 'foreign' is 'f'.
'una'+'f'+'raid'='UNAFRAID'
(Other definitions for unafraid that I've seen before include "Feeling no fear" , "Not timorous or apprehensive" , "not daunted" , "Not cowed" , "game" .)