Reserve a cape worn by retiring jester (9)
I believe the answer is:
aloofness
'reserve' is the definition.
(thesaurus)
'a cape worn by retiring jester' is the wordplay.
'cape' becomes 'ness' ('ness' can be a synonym of 'cape'**).
'worn by' is an insertion indicator (I've seen this in other clues).
'retiring' is a reversal indicator.
'jester' becomes 'fool' ('fool' can be a synonym of 'jester').
'fool' written backwards gives 'loof'.
'a'+'ness'='aness'
'aness' going around 'loof' is 'ALOOFNESS'.
(Other definitions for aloofness that I've seen before include "Standoffish air" , "Indifference by personal withdrawal" , "Indifference, reserve" , "Cool and distant manner" , "Haughty nature" .)