Real flat bombs, essentially (5,3)
I believe the answer is:
after all
'essentially' is the definition.
'real flat bombs' is the wordplay.
'bombs' indicates an anagram.
'real'+'flat'='realflat'
'realflat' with letters rearranged gives 'AFTER ALL'.
(Other definitions for after all that I've seen before include "comes in last?" , "In spite of everything" , "Where most dictionary entries are" , "'In spite of what's been said and done (5,3)'" , "most importantly" .)