Oh, so it is! Wasn't on the wavelength of that 'top' somehow -- not even one of the toughest clues -- just a mental block I guess! Many thanks mattrom.
PS I should have added a shout-out to 42a. What a fine bit of clueing. I'm still chortling. There's a name for contranyms: I shall now look out for, er, mutual contra-synonyms? Brilliant :-)