Large dress must be altered, no matter what (10)
I believe the answer is:
regardless
'no matter what' is the definition.
(thesaurus)
'large dress must be altered' is the wordplay.
'must be altered' indicates anagramming the letters.
'large'+'dress'='largedress'
'largedress' anagrammed gives 'REGARDLESS'.
(Other definitions for regardless that I've seen before include "Indifferent - come what may" , "CARRY ON FILM" , "in spite of everything" , "Despite the prevailing circumstances" , "Heeding no one - one of the 14 films" .)