Subject to the first person (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'the first person' is the wordplay.
'the first' becomes 't' (hst letter of 'the').
'person' becomes 'heme' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'heme'='THEME'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "basis for variations" , "Subject matter, motif" , "thing under discussion" , "Motif - 4 across" , "Topic, leitmotif" .)