Little part of chariot axle (4)
I believe the answer is:
'little part' is the definition.
(I've seen this before)
'of chariot axle' is the wordplay.
'of' says the answer is hidden in the clue.
'IOTA' can be found hidden inside 'chariot axle'.
(Other definitions for iota that I've seen before include "Grain" , "Smidgeon" , "'Very small amount, jot (4)'" , "Ninthe letter of the Greek alphabet" , "(Not one) little bit" .)