5a Spliffs - 'if' (provided, in) 's(ome) p(rosthetic) l(imbs) f(eel) s(tiff)' (at first)
6d Lucubrate - 'l(arge)' + 'Cu' (Copper, in) 'U(niform) b(ogus; heading)' + 'rate' (judge)
10a Disconcerts - 'discos' (entertaining) 'n(orthern) cert(ainty; banker)'
26a Old bean - 'lo' (look, around) + anag 'and be'