I suspect this version of 'clap' is a northern usage along the same lines as 'slap' as seen in phrases such as "slap-bang in the middle", meaning 'exactly' , 'completely', 'totally' etc. A clap and a slap are more or less synonymous in some circumstances, particularly in northern dialects.
My father in law (from Sheffield) used the expression 'clock cold', not just for food, but also water - eg the hot water tank being 'clock cold'. Mind you, he moved around a bit - could also come from Kent or Cumbria.
Thanks for your replies. I`ve heard the expression many times but never questioned it`s origins until my daughter asked recently. I`ve never heard of `clock cold`. I would imagine that most people would say `stone cold`.