Figure ending in prison: you upset about that? (8)
I believe the answer is:
thousand
'figure' is the definition.
(I've seen this before)
'ending in prison you upset about that?' is the wordplay.
'ending in' suggests the final letters.
'you' becomes 'thou' (I've seen this before).
'upset' becomes 'sad' (I've seen this before).
'about that?' indicates putting letters inside.
The last letter of 'prison' is 'n'.
'thou'+'sad'='thousad'
'n' placed into 'thousad' is 'THOUSAND'.
(Other definitions for thousand that I've seen before include "-- Island dressing" , "a fair amount?" , "See 19 Across" , "Figure" , "Ten cubed" .)