Wash first of nightclothes in advance (5)
I believe the answer is:
rinse
'wash' is the definition.
(rinse is a kind of wash)
'first of nightclothes in advance' is the wordplay.
'first of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'advance' becomes 'rise' ('rise' can be a synonym of 'advance'**).
The initial letter of 'nightclothes' is 'n'.
'n' going within 'rise' is 'RINSE'.
(Other definitions for rinse that I've seen before include "Remove soap with clean water" , "Wash without using soap" , "Wash lightly without soap" , "Wash - siren (anag)" , "Colour" .)