Paper's introduction to topical subject (6)
I believe the answer is:
tissue
'paper's' is the definition.
(tissue paper is a kind of paper)
'introduction to topical subject' is the wordplay.
'introduction to' says to take the initial letters (I've seen 'introductions to' mean this).
'subject' becomes 'issue' ('issue' can be a synonym of 'subject').
The first letter of 'topical' is 't'.
't'+'issue'='TISSUE'
(Other definitions for tissue that I've seen before include "Web; fabric" , "Cloth" , "Disposable handkerchief" , "cell block?" , "Thin, absorbent paper" .)