Girl showing ambition, on the up (3)
I believe the answer is:
mia
'girl' is the definition.
'ambition on the up' is the wordplay.
'ambition' becomes 'aim' (aim can mean an ambition or goal).
'on the up' shows that the letters should be reversed in order (in a down clue, letters go up).
'aim' written backwards gives 'MIA'.
'showing' is the link.
(Other definitions for mia that I've seen before include "- - Farrow, actress" , "girl" .)