Heard flag formed part of a car (4)
I believe the answer is:
tyre
'part of a car' is the definition.
'heard flag' is the wordplay.
'heard' shows a homophone (sound like).
'flag' becomes 'tire' (I've seen this in other clues).
'tire' is a homophone of 'TYRE'.
'formed' acts as a link.
(Other definitions for tyre that I've seen before include "Biblical town" , "Rubber wheel casing" , "Rubber wheel-ring" , "Ancient (and modern) port - rim for wheel" , "Wheel cover, may be tubeless" .)