Upper-class idlers, retiring, occasionally exercised (4)
I believe the answer is:
used
'exercised' is the definition.
(thesaurus)
'upper-class idlers retiring' is the wordplay.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'idlers' becomes 'des' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'retiring' shows that the letters should be reversed in order.
'des' written backwards gives 'sed'.
'u'+'sed'='USED'
'occasionally' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for used that I've seen before include "with a previous owner" , "Old" , "Availed oneself of" , "Hand-me-down" , "Accustomed - exploited" .)