I think you are correct C (about) + RICKETS (old childhood disease).
Crickets meaning ‘silence’ (not a word) i.e. just the sound of chirping, comes from the film industry it seems. I can’t find it in any dictionary though!
Oh yes, I've realised that vaguely American phraselology is part of the FT Magazine Crossword Setter's vocabulary. Probably get "You Do The Math" next... Or "My Bad"..... Jeesh