Following everything, most importantly (5,3)
I believe the answer is:
after all
'most importantly' is the definition.
(I've seen this before)
'following everything' is the wordplay.
'following' becomes 'after' (following can mean after or as a result of).
'everything' becomes 'all' (all things).
'after'+'all'='AFTER ALL'
(Other definitions for after all that I've seen before include "Where most dictionary entries are" , "In spite of everything" , "in the final reckoning?" , "when push comes to shove" , "comes in last?" .)