12a Christie - 'CH' (Switzerland) + 'ris(e[ast])' + 'tie' (draw)
14a 'sum' (problem) + 'MD' (about) 'a rise' (change in pitch)
16a Fuss - 'f(emale)' + 'US' + 's(econd)'
28a Natural - double meaning
25a & 15d Gold medallist (Lord Coe, perhaps) - 'old' (decrepit) + (Stratfor)d (end of, both in) anag 'itll + games'