Understand secure subjects given by guru (3,3,7)
I believe the answer is:
get the message
'understand' is the definition.
(I've seen this before)
'secure subjects given by guru' is the wordplay.
'secure' becomes 'get' (secure can mean to get or obtain).
'subjects' becomes 'themes' ('theme' can be a synonym of 'subject').
'given by' says to put letters next to each other.
'guru' becomes 'sage' (synonyms).
'get'+'themes'+'sage'='GET THE MESSAGE'
(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" .)