Hot rod features -- one's unreadable (5,4)
I believe the answer is:
poker face
'one's unreadable' is the definition.
(a poker face can't be read or understood)
'hot rod features' is the wordplay.
'hot rod' becomes 'poker' (as in a poker in a fire).
'features' becomes 'face'.
'poker'+'face'='POKER-FACE'
(Other definitions for poker face that I've seen before include "inscrutable countenance" , "Expressionless visage, useful when playing cards" , "Impassive visage as employed by card gambler" , "Card player's deadpan expression" , "unreadable profile" .)