Contention if rest is disturbed (6)
I believe the answer is:
strife
'contention' is the definition.
(thesaurus)
'if rest is disturbed' is the wordplay.
'is disturbed' indicates an anagram.
'if'+'rest'='ifrest'
'ifrest' anagrammed gives 'STRIFE'.
(Other definitions for strife that I've seen before include "'Discord, conflict (6)'" , "Trouble and discord" , "Fighting" , "Trouble and conflict" , "A Cockney's wife is his trouble and .....(60" .)