Rule a girl unfit to be a soldier (9)
I believe the answer is:
guerrilla
'a soldier' is the definition.
(I've seen this before)
'rule a girl unfit' is the wordplay.
'unfit' indicates anagramming the letters.
'rule'+'a'+'girl'='ruleagirl'
'ruleagirl' with letters rearranged gives 'GUERRILLA'.
'to be' acts as a link.
(Other definitions for guerrilla that I've seen before include "Underground fighter" , "Irregular fighter" , "Such tactics" , "One fighting" , "Person who fights as one of small independent group" .)