Plain like old tyre? (4)
I believe the answer is:
bald
'plain' is the definition.
(both can mean unadorned)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for bald that I've seen before include "Without hair" , "With no effort to conceal" , "Lacking hair on the scalp" , "Not covered with hair" , "Follicly challenged?" .)