£500 to trade in mischief (6,8)
I believe the answer is:
monkey business
'mischief' is the definition.
(I know that monkey business is a type of mischief)
'ps500 to trade' is the wordplay.
'pse00' becomes 'monkey' (I can't justify this - if you can you should give a lot more credence to this answer).
'to trade' becomes 'business' (I've seen this before).
'monkey'+'business'='MONKEY BUSINESS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for monkey business that I've seen before include "larking about" , "Marx Brothers film" , "Mischievous behaviour" , "fiddle" , "may be amusingly projected" .)