So, it turns out that
22 Across : Law, cheating it (anag) (5,8)
is just an anagram, and the solution - Wigan Athletic - has no obvious connection with illegal activity. That made it pretty plain that
15 Down : Knocking sound (3-3)
would be 'tat-tat' as in 'rat-a-tat-tat'. I was stumped by
12 Down : Celebration (8)
for an eternity or two, but I realised they wanted, as a solution to a different clue, not 'leitmotif', but rather 'leitmotiv', the v at the end of which meshed beautifully with the sixth letter of 'festival'.