Mixture of 50% recycled oil (4)
I believe the answer is:
olio
'mixture' is the definition.
(olio is a type of stew)
'of 50 recycled oil' is the wordplay.
'50' indicates taking half.
'recycled' indicates an anagram.
'of' halved is 'o'.
'oil' is an anagram of 'lio'.
'o'+'lio'='OLIO'
(Other definitions for olio that I've seen before include "Savoury dish of different types of meat and vegetable" , "Variety" , "Collection of miscellany" , "Mixture" .)