So, to free her, let loose (9)
I believe the answer is:
therefore
'so' is the definition.
(I've seen this before)
'to free her let loose' is the wordplay.
'let loose' indicates anagramming the letters.
'to'+'free'+'her'='tofreeher'
'tofreeher' anagrammed gives 'THEREFORE'.
(Other definitions for therefore that I've seen before include "Consequently - for that reason" , "has a triangular formation in maths?" , "It follows" , "Thus" , "Ergo" .)