Old Nick wouldn't stand for one! (5)
I believe the answer is:
satan
'old nick' is the definition.
(I know that old nick can be written as Satan)
'wouldn't stand for one' is the wordplay.
'wouldn't stand' becomes 'sat' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other).
'one' becomes 'an' (an apple is one apple).
'sat'+'an'='SATAN'
Can you help me to learn more?
(Other definitions for satan that I've seen before include "Fallen archangel" , "Old Nick, Lucifer or Beelzebub" , "Prince of Darkness" , "Apollyon" , "arch-fiend" .)