Start to cook Jack Sprat's dinner, then wash up (5)
I believe the answer is:
clean
'wash up' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for clean that I've seen before include "Second part of 18" , "Squeaky?" , "thoroughly" , "Without stain" , "Squeakily hygienic?" .)