Extremely reliable vehicle - one holding things back? (9)
I believe the answer is:
retractor
'one holding things back?' is the definition.
(someone or something who retracts)
'extremely reliable vehicle' is the wordplay.
'extremely' suggests removing the centre.
'vehicle' becomes 'tractor' (tractor is a kind of vehicle).
'reliable' with its centre taken out is 're'.
're'+'tractor'='RETRACTOR'
(Other definitions for retractor that I've seen before include "It must pull back" , "that springs back in reverse?" , "Instrument" , "Muscle" , "One withdraws" .)