This subject comes first, I see (5)
I believe the answer is:
topic
'this subject' is the definition.
(as in a topic for discussion)
'first i see' is the wordplay.
'first' becomes 'top' (top can mean the best or first).
'see' becomes 'C' (the word for the letter, according to Chambers).
'top'+'i'+'c'='TOPIC'
'comes' acts as a link.
(Other definitions for topic that I've seen before include "'Theme, subject (5)'" , "Question" , "Talking point" , "Something to discuss" , "Subject (for conversation, etc)" .)