Thus, thunderer has broken free agent's heart (9)
I believe the answer is:
therefore
'thus' is the definition.
(I know that thus can be written as therefore)
'thunderer has broken free agent's heart' is the wordplay.
'thunderer' becomes 'thor' (I have seen 'The Thunderer' mean 'thor' so perhaps 'thunderer' could also mean 'thor').
'has' indicates putting letters inside.
'broken' is an anagram indicator.
'heart' means to look at the middle letters.
The central letter of 'agent' is 'e'.
'free' with letters rearranged gives 'eref'.
'thor' going around 'eref' is 'therefor'.
'therefor'+'e'='THEREFORE'
(Other definitions for therefore that I've seen before include "Ergo" , "It follows" , "Thus" , "as result" , "has a triangular formation in maths?" .)