Tailless cat rocks rock (6)
I believe the answer is:
pumice
'rock' is the definition.
(pumice is a kind of rock)
'tailless cat rocks' is the wordplay.
I cannot really understand how this works, but
'rocks' could be 'ice' (both can informally mean diamonds) and 'ice' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pumice that I've seen before include "Light, porous volcanic rock" , "Light porous stone used as polishing powder" , "Piece of volcanic rock used as an abrasive" , "Volcanic-rock scourer" , "Porous lava" .)