Tacitly agreed to break heartless spy — no difficulty (10)
I believe the answer is:
simplicity
'no difficulty' is the definition.
(simplicity is the opposite of difficulty)
'tacitly agreed to break heartless spy' is the wordplay.
'tacitly agreed' becomes 'implicit' (implicit means tacitly agreed or implied).
'to break' means one lot of letters goes inside another (inserted letters break into the word).
'heartless' means to remove the middle letters.
'spy' with its centre removed is 'sy'.
'implicit' going inside 'sy' is 'SIMPLICITY'.
(Other definitions for simplicity that I've seen before include "Facility, easiness" , "Couldn't be easier!" , "Lack of affectation or pretence" , "Clarity" , "Ease" .)