Slip up badly in summer month, backing freedom fighter (9)
I believe the answer is:
guerrilla
'freedom fighter' is the definition.
(as in guerrilla fighters)
'slip up badly in summer month backing' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'in' means one lot of letters goes inside another.
'summer month' becomes 'aug' (short for August).
'backing' says the letters should be written in reverse.
'aug' back-to-front is 'gua'.
'err'+'ill'='errill'
'errill' placed within 'gua' is 'GUERRILLA'.
(Other definitions for guerrilla that I've seen before include "Person who fights as one of small independent group" , "Member of politically-motivated fighting group" , "Irregular fighter" , "Politically motivated fighter" , "One fighting" .)