Get up and get undressed to get lighter (5)
I believe the answer is:
barge
'lighter' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for barge that I've seen before include "Canal-boat" , "Roughly push one's way" , "Freight vessel" , "Narrow boat" , "Flat-bottomed canal boat" .)