As a subject, the best, I see (5)
I believe the answer is:
topic
'as a subject the' is the definition.
I can't judge whether this definition defines the answer.
'best i see' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'see' becomes 'c' (the word for the letter, according to Chambers).
'top'+'i'+'c'='TOPIC'
Can you help me to learn more?
(Other definitions for topic that I've seen before include "might this be hot?" , "Theme (of, eg, a talk)" , "Matter for an argument" , "one discussed" , "Subject matter, theme" .)