Make possible arrest, breaking Helen's heart (6)
I believe the answer is:
enable
'make possible' is the definition.
(I've seen this before)
'arrest breaking helen's heart' is the wordplay.
'arrest' becomes 'nab' ('nab' can be a synonym of 'arrest').
'breaking' is an insertion indicator.
'heart' indicates the central letters.
The middle letters of 'helen' are 'ele'.
'nab' placed into 'ele' is 'ENABLE'.
(Other definitions for enable that I've seen before include "Make it possible for someone to do something" , "Capacitate" , "Activate" , "Make possible using baleen" , "Make possible, give the means to do something" .)