Machine part is a faucet favourite (6)
I believe the answer is:
tappet
'machine part' is the definition.
(part of an engine)
'faucet favourite' is the wordplay.
'faucet' becomes 'tap' (tap is a kind of faucet).
'favourite' becomes 'pet' (synonyms).
'tap'+'pet'='TAPPET'
'is a' acts as a link.
(Other definitions for tappet that I've seen before include "Public protest" , "motor part" , "Lever to transmit motion from one part to another, in car engine say" , "Apt pet for valve operator" , "Moving part in an internal combustion engine" .)