Marx among others once mucked around with duck film (9)
I believe the answer is:
economist
'marx among others' is the definition.
I can't tell whether this defines the answer.
'mucked around with duck film' is the wordplay.
'mucked around' becomes 'econ' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'film' becomes 'mist' (to film over is to mist or fog over).
'econ'+'o'+'mist'='ECONOMIST'
'once' is the link.
Can you help me to learn more?
(Other definitions for economist that I've seen before include "Expert on finances" , "Comes into (anag)" , "Money supply expert" , "Smith, perhaps" , "government adviser, possibly" .)