A slim, upright figure (3)
I believe the answer is:
one
This could be a double definition.
'a' is the first definition.
'slim upright figure' is the second definition.
'one' can be an answer for 'figure' (I've seen this before). I am unsure of the 'slim upright' bit.
Can you help me to learn more?
(Other definitions for one that I've seen before include "Smallest whole number" , "SECOND PART OF 16" , "Hour after midnight" , "Sole; number" , "Half of two" .)