Best chapter about current theme (5)
I believe the answer is:
topic
'theme' is the definition.
('topic' can be a synonym of 'theme')
'best chapter about current' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'chapter' becomes 'c'.
'about' means one lot of letters go next to another.
'current' becomes 'i' (symbol for electric current).
'c' after 'i' is 'ic'.
'top'+'ic'='TOPIC'
(Other definitions for topic that I've seen before include "one discussed" , "Subject (of a talk, etc)" , "Subject matter of a conversation or discussion" , "Question" , "Talking point" .)