With respect to engineers, it's classified (3-6)
I believe the answer is:
top-secret
'classified' is the definition.
(I've seen this before)
'with respect to engineers' is the wordplay.
'with' is a charade indicator (letters next to each other).
'engineers' is an anagram indicator (I've seen 'engineer' mean this).
'respect' next to 'to' is 'respectto'.
'respectto' anagrammed gives 'TOP-SECRET'.
'it's' acts as a link.
(Other definitions for top-secret that I've seen before include "Most confidential , not to be leaked on any account" , "Extremely restricted (information)" , "Highly classified" , "Erect post (anag)" , "Very hush-hush" .)