Become wise, absorbing subjects to understand (3,3,7)
I believe the answer is:
get the message
'understand' is the definition.
(I've seen this before)
'become wise absorbing subjects' is the wordplay.
'become' becomes 'get' ('get' can be a synonym of 'become').
'wise' becomes 'sage' ('sage' can be similar in meaning to 'wise').
'absorbing' means one lot of letters goes inside another.
'subjects' becomes 'themes' ('theme' can be a synonym of 'subject'**).
'get'+'sage'='getsage'
'getsage' enclosing 'themes' is 'GET THE MESSAGE'.
'to' acts as a link.
(Other definitions for get the message that I've seen before include "Understand (colloq.)" , "Understand what is implied by a remark or action" , "Understand what is meant" , "Access text on phone" , "Cotton on" .)