Love mum's ace raw tripe (5)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

O + MA’S A(ce). Nho (so, word no.9), it means “the third compartment of the stomach of ruminants, between the reticulum and abomasum.” My Sheffield mum and granny both loved tripe. But definitely not me. The tripe is only the lining of the stomach, so not a very precise definition maybe..