Hence setting hero free behind junction (9)
I believe the answer is:
therefore
'hence' is the definition.
(I know that hence can be written as therefore)
'setting hero free behind junction' is the wordplay.
'setting' indicates anagramming the letters.
'behind' means one lot of letters go next to another.
'junction' becomes 't'.
'hero'+'free'='herofree'
'herofree' anagrammed gives 'herefore'.
'herefore' after 't' is 'THEREFORE'.
(Other definitions for therefore that I've seen before include "In consequence" , "Thus" , "as result" , "has a triangular formation in maths?" , "It follows" .)