Reduction of stewed bat eaten with first bit of Marmite (9)
I believe the answer is:
abatement
'reduction' is the definition.
(abatement is a kind of reduction)
'stewed bat eaten with first bit of marmite' is the wordplay.
'stewed' is an anagram indicator.
'with' means one lot of letters goes inside another.
'first bit of marmite' becomes 'm' (1st letter of 'marmite').
'bat'+'eaten'='bateaten'
'bateaten' anagrammed gives 'abateent'.
'abateent' going around 'm' is 'ABATEMENT'.
'of' is the link.
(Other definitions for abatement that I've seen before include "Subsidence" , "Lessening in intensity or amount" , "mitigation" , "relief" .)