Working CEO's left out of Milton Keynes, perhaps (9)
I believe the answer is:
economist
'keynes perhaps' is the definition.
(keynes is a kind of economist)
'working ceo's left out of milton' is the wordplay.
'working' indicates an anagram.
'left' becomes 'l' (common abbreviation).
'out of' indicates named letters should be taken away ('out of' can mean no longer having).
'milton' with 'l' taken away is 'miton'.
'ceos'+'miton'='ceosmiton'
'ceosmiton' is an anagram of 'ECONOMIST'.
(Other definitions for economist that I've seen before include "Expert on finances" , "publication" , "Financial specialist" , "Expert in all aspects of wealth" , "Smith, perhaps" .)