Understand and become wise about topics (3,3,7)
I believe the answer is:
get the message
'understand' is the definition.
(I've seen this before)
'become wise about topics' is the wordplay.
'become' becomes 'get' ('get' can be a synonym of 'become').
'wise' becomes 'sage' ('sage' can be similar in meaning to 'wise').
'about' is a charade indicator (letters next to each other).
'topics' becomes 'themes' ('theme' can be a synonym of 'topic'**).
'sage' put after 'themes' is 'themessage'.
'get'+'themessage'='GET THE MESSAGE'
'and' acts as a link.
(Other definitions for get the message that I've seen before include "Understand (colloq.)" , "Cotton on" , "Understand what is implied by a remark or action" , "Access text on phone" , "Understand what is meant" .)