Deceitful conduct of male running major concern (6,8)
I believe the answer is:
monkey business
'deceitful conduct' is the definition.
The definition suggests a singular noun which matches the answer.
'male running major concern' is the wordplay.
'male' becomes 'm' (common abbreviation).
'running' becomes 'on' (eg when a machine is running it is on).
'major' becomes 'key' (I've seen this in another clue).
'concern' becomes 'business' (concern can mean a business or company).
'm'+'on'+'key'+'business'='MONKEY BUSINESS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for monkey business that I've seen before include "fiddle" , "larking about" , "Marx Brothers film" , "mischief" , "Mischievous behaviour" .)