Remove jacket to smarten up jumper (3)
I believe the answer is:
roo
'jumper' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for roo that I've seen before include "Kanga's child (Milne)" , "Hopper" , "short jumper" , "Young marsupial" , "Diminutive inhabitant of Hundred Acre Wood" .)