Scornful humour by that lady in Gala opening (9)
I believe the answer is:
withering
'scornful' is the definition.
(thesaurus)
'humour by that lady in gala opening' is the wordplay.
'humour' becomes 'wit' (synonyms).
'by' says to put letters next to each other.
'that lady' becomes 'her' ('her' refers to a woman).
'opening' indicates taking the first letters.
The initial letter of 'gala' is 'g'.
'wit'+'her'+'in'+'g'='WITHERING'
(Other definitions for withering that I've seen before include "Supercilious" , "Devastating" .)