... altogether popular with advanced students (2,3)
I believe the answer is:
in all
'altogether' is the definition.
(I know that altogether can be written as in all)
'popular with advanced students' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'with' says to put letters next to each other.
'advanced' becomes 'a' (abbreviation).
'students' becomes 'LL' (two abbreviations for learner).
'in'+'a'+'ll'='IN ALL'
(Other definitions for in all that I've seen before include "Altogether" , "Completely" , "Summarising" , "Everything considered" , "after considering everything" .)