The day before starling went off for good (11)
I believe the answer is:
everlasting
'good' is the definition.
(thesaurus)
'the day before starling went off' is the wordplay.
'the day before' becomes 'eve' (eve is the night before something).
'went off' indicates an anagram.
'starling' anagrammed gives 'rlasting'.
'eve'+'rlasting'='EVERLASTING'
'for' is the link.
(Other definitions for everlasting that I've seen before include "describes Wonka's gobstoppers" , "Unceasing" , "description of the rest Romeo set up here?" , "Timeless" , "Unending" .)