Old fool conserving energy spotted cat (6)
I believe the answer is:
ocelot
'spotted cat' is the definition.
(wild cat that has spots)
'old fool conserving energy' is the wordplay.
'old' becomes 'o' (abbreviation).
'fool' becomes 'clot' (I've seen this in other clues).
'conserving' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mcl).
'clot' enclosing 'e' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "Fur" , "Big cat" , "Wild creature" , "South and Central American wildcat" , "Feline mammal" .)