One involved in row last to leave washroom (7)
I believe the answer is:
sculler
'one involved in row' is the definition.
(someone on a rowing boat)
'last to leave washroom' is the wordplay.
'last to leave' means to remove the last letter.
'washroom' becomes 'scullery' (room for washing dishes).
'scullery' with its last letter removed is 'SCULLER'.
(Other definitions for sculler that I've seen before include "One rowing" , "One has a row" , "22 with two blades" .)