Bargain with tough heartless sort of bomber (7)
I believe the answer is:
stealth
'sort of bomber' is the definition.
(as in a stealth bomber)
'bargain with tough heartless' is the wordplay.
'bargain' becomes 'steal' (synonyms).
'with' says to put letters next to each other.
'heartless' says to hollow out the word (remove centre letters).
'tough' with its middle taken out is 'th'.
'steal'+'th'='STEALTH'
(Other definitions for stealth that I've seen before include "Cautious and surreptitious action or movement" , "The last (anag.)" , "Cats display this" , "Secretiveness, furtiveness" , "By which taxes go up" .)