Working model directs film about alien (5,3)
I believe the answer is:
train set
'working model' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'directs film about alien' is the wordplay.
I cannot quite see how this works, but
'directs' could be 'trains' ('train' can be a synonym of 'direct') and 'trains' is located in the answer.
'film' could be 'et' (E.T. the Extra-Terrestrial) and 'et' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for train set that I've seen before include "Nitrates (anag) - Hornby product?" , "scaled-down transport system" , "Number of tracked items" , "Model tracking" , "little loco with this?" .)