Start dressing, say (6)
I believe the answer is:
source
'start' is the definition.
(thesaurus)
'dressing say' is the wordplay.
'dressing' becomes 'sauce' (I've seen this before).
'say' shows a homophone (sound like).
'sauce' is a homophone of 'SOURCE'.
(Other definitions for source that I've seen before include "Origin; head of stream" , "Origin or starting point" , "'Origin, root (6)'" , "Spring, fountain head" , "Origin, starting-point" .)