Happy medium may end after blunder (5)
I believe the answer is:
merry
'happy' is the definition.
(thesaurus)
'medium may end after blunder' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'end' indicates one should take the final letters.
'after' says to put letters next to each other.
'blunder' becomes 'err' (to err is to make a mistake or blunder).
The final letter of 'may' is 'y'.
'y' put after 'err' is 'erry'.
'm'+'erry'='MERRY'
(Other definitions for merry that I've seen before include "Fjull of fun and gaiety" , "Jovial" , "Enlivened by drink" , "Drunk - happy" , "Seasons 24" .)