A cut introduces mean ration (9)
I believe the answer is:
allowance
'ration' is the definition.
(allowance is a kind of ration)
'a cut introduces mean' is the wordplay.
'cut' becomes 'lance' (I've seen this before**).
'introduces' indicates putting letters inside.
'mean' becomes 'low' (I've seen this before).
'lance' placed around 'low' is 'llowance'.
'a'+'llowance'='ALLOWANCE'
(Other definitions for allowance that I've seen before include "Allotted share" , "Fixed sum given regularly" , "Money granted for a purpose" , "Money for agreed expenses" , "Maintenance" .)