Rodent or sloth accepting painful condition (6)
I believe the answer is:
agouti
'rodent' is the definition.
(agouti is a kind of rodent)
'sloth accepting painful condition' is the wordplay.
'sloth' becomes 'ai' (ai is a kind of sloth).
'accepting' indicates putting letters inside (inserted letters are accepted).
'painful condition' becomes 'gout'.
'ai' enclosing 'gout' is 'AGOUTI'.
'or' is the link.
(Other definitions for agouti that I've seen before include "Burrowing rodent - IOU tag (anag)" , "Guinea pig-like rodent" , "animal" , "Caribbean rodent" , "Large rodent" .)