Green train is not heading for Milton Keynes? (9)
I believe the answer is:
economist
'keynes?' is the definition.
(keynes is a kind of economist)
'green train is not heading for milton' is the wordplay.
'green' becomes 'eco' (abbreviation for ecological).
'train' indicates an anagram (to train can mean to grow something into a shape).
'heading for' indicates taking the first letters.
The first letter of 'milton' is 'm'.
'is'+'not'+'m'='isnotm'
'isnotm' anagrammed gives 'nomist'.
'eco'+'nomist'='ECONOMIST'
(Other definitions for economist that I've seen before include "Comes into (anag)" , "John Maynard Keynes for instance" , "Friedman, say" , "Smith, perhaps" , "He studies money etc" .)