Best to escape highest level (3,6)
I believe the answer is:
top flight
'highest level' is the definition.
(I've seen this before)
'best to escape' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'to escape' becomes 'flight' (synonyms).
'top'+'flight'='TOP-FLIGHT'
(Other definitions for top flight that I've seen before include "Steps to attic" , "stairs leading to the attic?" , "so good?" , "Superior" , "This could afford access to penthouse" .)