To coppers, I see, it's an interesting subject (5)
I believe the answer is:
topic
'an interesting subject' is the definition.
'topic' can be an answer for 'subject' (as in a topic for discussion). I am not certain of the 'an interesting' bit.
'to coppers i see' is the wordplay.
'coppers' becomes 'p'.
'see' becomes 'C' (the word for the letter, according to Chambers).
'to'+'p'+'i'+'c'='TOPIC'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme of discourse" , "one discussed" , "A subject for conversation or debate" , "might this be hot?" , "Subject (of speech, etc)" .)