Brother exchanges the old trade for mischief (6,8)
I believe the answer is:
monkey business
'mischief' is the definition.
(I've seen this before)
'brother exchanges the old trade' is the wordplay.
'brother' becomes 'monk' (brother can mean a monk).
'exchanges the old' becomes 'ey' (I can't justify this - if you can you should give a lot more credence to this answer).
'trade' becomes 'business' (I've seen this before).
'monk'+'ey'+'business'='MONKEY BUSINESS'
'for' is the link.
Can you help me to learn more?
(Other definitions for monkey business that I've seen before include "may be amusingly projected" , "Marx Brothers film" , "shady deal" , "fiddle" , "mischief" .)