Possibly upright girl (6)
I believe the answer is:
joanna
This clue is a double definition.
'possibly upright' is the first definition.
(Joanna is rhyming slang for a piano)
'girl' is the second definition.
(Other definitions for joanna that I've seen before include "Girl's name, piano" , "east end piano" , "Piano (rhyming slang)" , "One's played" .)