Subject matter ran out of alienated earthmen (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'matter ran out of alienated earthmen' is the wordplay.
I cannot really see how this works, but
'matter' could be 'heme' (heme is a kind of matter) and 'heme' is found in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for theme that I've seen before include "issue" , "notedly subject to variation" , "Melody on which variations are constructed" , "essay" , "sort of tune" .)