Pact, at heart, what’s needed for binding agreement (6)
I believe the answer is:
accord
'for binding agreement' is the definition.
(I've seen this before)
'pact at heart what's' is the wordplay.
'at heart' means to look at the middle letters.
'what's' becomes 'cord' (I can't explain this - if you can you should believe this answer much more).
The middle letters of 'pact' are 'ac'.
'ac'+'cord'='ACCORD'
'needed' is the link.
(Other definitions for accord that I've seen before include "Harmony, concurrence" , "binding agreement" , "Match" , "Harmonize" , "Unanimity" .)