This month star's left out as a substitute (7)
I believe the answer is:
instead
'as a substitute' is the definition.
(I've seen this before)
'this month star's left' is the wordplay.
I cannot quite see how this works, but
'month' could be 'inst' (I've seen this before) and 'inst' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'out' acts as a link.
Can you help me to learn more?
(Other definitions for instead that I've seen before include "As a substitute" , "as an alternative option" , "In lieu" , "Stained (anag)" , "Rather - detains (anag)" .)