Flog the elderly? That's the limit (9)
I believe the answer is:
threshold
'the limit' is the definition.
('threshold' can be a synonym of 'limit')
'flog the elderly?' is the wordplay.
'flog' becomes 'thresh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the elderly?' becomes 'old' (I've seen this before).
'thresh'+'old'='THRESHOLD'
'that's' is the link.
Can you help me to learn more?
(Other definitions for threshold that I've seen before include "Point of entry" , "Lower limit" , "Doorway, entrance" , "The bride may be carried across this door bottom" , "Beginning" .)