A suitable insulating timber (3)
I believe the answer is:
oak
'timber' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'a suitable insulating' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'suitable' could be 'ok' and 'ok' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oak that I've seen before include "Building timber" , "Hard durable wood of a deciduous tree" , "The King of the Forest" , "Deciduous forest tree" , "From little acorns does it grow" .)