Rue a girl's frolicking with resistance fighter (9)
I believe the answer is:
irregular
'fighter' is the definition.
(member of an irregular fighting force)
'rue a girl's frolicking with resistance' is the wordplay.
'frolicking with' indicates an anagram.
'resistance' becomes 'r' (electronics abbreviation).
'rue'+'a'+'girl'='rueagirl'
'rueagirl' anagrammed with 'r' can make 'IRREGULAR'.
(Other definitions for irregular that I've seen before include "Abnormal" , "one of Holmes's Baker Street urchins" , "Contrary to a rule or convention" , "Erratic" , "commonest verbs?" .)