It upset socialist, weary (5)
I believe the answer is:
tired
'weary' is the definition.
(similar in meaning)
'it upset socialist' is the wordplay.
'upset' is a reversal indicator.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'it' in reverse letter order is 'ti'.
'ti'+'red'='TIRED'
(Other definitions for tired that I've seen before include "Weary, jaded" , "Done in" , "Played out" , "Spent" , "Physically or mental weary" .)