You once include Mike in subject (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'you once include mike' is the wordplay.
'you once' becomes 'thee' (historical second person pronoun).
'include' is an insertion indicator.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'thee' enclosing 'm' is 'THEME'.
'in' acts as a link.
(Other definitions for theme that I've seen before include "basis for variations" , "Subject; recurring idea" , "Recurrent unifying idea" , "Main idea or topic" , "Burden" .)