Breaking the rule, is giving a key to the rest (7)
I believe the answer is:
leisure
'the rest' is the definition.
(leisure is a kind of rest)
'breaking the rule is giving a key' is the wordplay.
'breaking' indicates putting letters inside.
'giving' is an anagram indicator (the letters give the letters for another word).
'a key' becomes 'e' (I've seen this in other clues).
'rule'+'is'='ruleis'
'ruleis' is an anagram of 'lisure'.
'lisure' going around 'e' is 'LEISURE'.
'to' is the link.
(Other definitions for leisure that I've seen before include "Use lire (anag)" , "Enjoyment of free time" , "Time to rest and enjoy" , "Period of relaxation" , "Convenient opportunity" .)