Defensive barrier - a six-letter word that appears now and again. Trouble is the usual/common three letter word, inside a two-letter word for 'reserve' and the usual abbreviation for 'base'. That's probably far too big a hint - sorry.
I have all but 10 (Head back ... starting E) and 23 (Borough ... starting L, M, N or O). The snag is that I've fitted everything else in and the checking letters don't allow any words which fit. Summat oop.