Discretionary supplement in tax rate, 50 per cent of loophole being abused (8,5)
I believe the answer is:
optional extra
'discretionary supplement' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'in tax rate 50 per cent of loophole being abused' is the wordplay.
'50 per cent of' suggests halving the letters.
'being abused' indicates anagramming the letters.
'loophole' with half the letters taken is 'loop'.
'in'+'tax'+'rate'+'loop'='intaxrateloop'
'intaxrateloop' is an anagram of 'OPTIONAL EXTRA'.
Can you help me to learn more?
(Other definitions for optional extra that I've seen before include "Non-obligatory item" , "not part of the package" , "It would be nice to have" , "Selectable add-on" , "Non-standard feature" .)