Unknown figure made pawn in poisonous matter (5)
I believe the answer is:
topic
'pawn in poisonous matter' is the definition.
'topic' can be an answer for 'matter' (synonyms). I am unsure of the 'pawn in poisonous' bit.
'unknown figure' is the wordplay.
'unknown' becomes 'topi' (I can't justify this - if you can you should believe this answer much more).
'figure' becomes 'c' (I've seen this in other clues).
'topi'+'c'='TOPIC'
'made' is the link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme (of, eg, a talk)" , "one discussed" , "Issue (for discussion)" , "Question" , "might this be hot?" .)