Cladding of marble: ideally, even, it could be gold (5)
I believe the answer is:
medal
'it could be gold' is the definition.
(as in a gold medal)
'cladding of marble ideally even' is the wordplay.
'cladding of' says to hollow out the word (remove centre letters) (letters which clad or cover the word).
'even' means one should take alternating letters (even-numbered letters are used).
'marble' with its centre taken out is 'me'.
The alternate letters of 'ideally' are 'dal'.
'me'+'dal'='MEDAL'
(Other definitions for medal that I've seen before include "Award for service" , "Metal disc awarded as prize, say in Olympics" , "Religious or commenorative metal piece" , "Prize disc for as for Olympic Games win" , "Coin-like reward for merit" .)