Ending is lean if altered (6)
I believe the answer is:
finale
'ending' is the definition.
(I know that finale is a type of ending)
'lean if altered' is the wordplay.
'altered' is an anagram indicator.
'lean'+'if'='leanif'
'leanif' with letters rearranged gives 'FINALE'.
'is' acts as a link.
(Other definitions for finale that I've seen before include "Last item of event" , "Ending" , "Denouement" , "Conclusion of a piece of music" , "Final section of a musical composition" .)