Exhaust manifold initially receding in value (5)
I believe the answer is:
merit
'value' is the definition.
(I've seen this in another clue)
'exhaust manifold initially receding' is the wordplay.
'exhaust' becomes 'tire' (synonyms).
'initially' suggests taking the first letters.
'receding' is a reversal indicator.
The first letter of 'manifold' is 'm'.
'tire'+'m'='tirem'
'tirem' written backwards gives 'MERIT'.
'in' is the link.
(Other definitions for merit that I've seen before include "Excellence that deserves to be honoured" , "Have a right" , "The M in OM" , "credit" , "Rate" .)