Concerning an attack (5)
I believe the answer is:
about
'concerning' is the definition.
(thesaurus)
'an attack' is the wordplay.
'an' becomes 'a'.
'attack' becomes 'bout' (I've seen this before**).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "Concerning (something)" , "Concerning or around" , "Concerning or in neighbourhood" , "Almost; around" , "On the topic of" .)