He met some other form of subject (5)
I believe the answer is:
theme
'form of subject' is the definition.
'theme' can be an answer for 'subject' (theme can mean a topic or subject). I am not certain of the 'form of' bit.
'he met some other' is the wordplay.
'some other' is an anagram indicator.
'he'+'met'='hemet'
'hemet' anagrammed gives 'THEME'.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Motif, topic" , "Thread, melody" , "Subject (on which person writes, speaks)" , "Motif or melody" , "(Writer's) topic" .)