Arranged to cart a tree, mostly chestnut (10)
I believe the answer is:
terracotta
'chestnut' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'arranged to cart a tree mostly' is the wordplay.
'arranged' indicates anagramming the letters.
'mostly' means to remove the last letter.
'to'+'cart'+'a'+'tree'='tocartatree'
'tocartatree' with its final letter removed is 'tocartatre'.
'tocartatre' is an anagram of 'TERRACOTTA'.
Can you help me to learn more?
(Other definitions for terracotta that I've seen before include "brownish-red colour" , "reddish colour" , "casting" , "statuette, perhaps" , "sculpture" .)