Ah. Thanks Crates, I'd been a bit thoughtless with 5a and got stuck with looking for some bomb aiming abbreviation rather than measures against bombarding. Should have spent more time leafing through Chambers and paying more attention to the many variations in spelling of the entry (5 six letter options!).
PS I think I'd better start a notebook of all the regular 2 or 3 letter clue components like that in 30d - they crop up pretty often in Azeds as well.