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' says the letters should be written backwards.
The initial letter of 'manifold' is 'm'.
'tire'+'m'='tirem'
'tirem' written backwards gives 'MERIT'.
'in' acts as a 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" .)