John, a vagrant in resort for bombing enemy within (6,5)
I believe the answer is:
trojan horse
'enemy within' is the definition.
(Greek soldiers hid within the Trojan horse)
'john a vagrant in resort for bombing' is the wordplay.
'vagrant' indicates anagramming the letters (I've seen this in other clues).
'in' indicates putting letters inside.
'for bombing' indicates an anagram.
'john'+'a'='johna'
'johna' is an anagram of 'janho'.
'resort' with letters rearranged gives 'trorse'.
'janho' placed within 'trorse' is 'TROJAN HORSE'.
(Other definitions for trojan horse that I've seen before include "Trap intended to undermine the enemy position (from Greek myth)" , "Investment aid" , "Damaging program" , "the Greeks aggressively piled out of" , "Malicious program" .)