To get irrational about success can mean pain (6)
I believe the answer is:
twinge
'mean pain' is the definition.
'twinge' can be an answer for 'pain' (twinge is a kind of pain). I am not certain of the 'mean' bit.
'to get irrational about success' is the wordplay.
'to get' becomes 'tg' (this might be a standard abbreviation of which I'm unaware).
'irrational' becomes 'E' (an irrational number).
'about' indicates putting letters inside.
'success' becomes 'win'.
'tg'+'e'='tge'
'tge' enclosing 'win' is 'TWINGE'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for twinge that I've seen before include "Sudden sharp pain" , "Slight pain (of conscience?)" , "A slight jab of pain (6)" , "Sudden darting pain" , "Sudden sharp feeling" .)