Tax father by the half year (4)
I believe the answer is:
paye
'tax' is the definition.
(I know that PAYE is a type of withholding)
'father by the half year' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'by' means one lot of letters go next to another.
'the half' suggests halving the letters.
'year' with half the letters taken is 'ye'.
'pa'+'ye'='PAYE'
(Other definitions for paye that I've seen before include "System of withholding tax" , "The working man's income tax system" , "Tax system for Joe Soap" , "Plain man's income tax system" , "The tax system for the working man" .)