That man behind alto, when sozzled, can’t stand (6)
I believe the answer is:
loathe
'can't stand' is the definition.
(I've seen this in another clue)
'that man behind alto when sozzled' is the wordplay.
'that man' becomes 'he' (he refers to a man).
'behind' is a charade indicator (letters next to each other).
'when sozzled' indicates anagramming the letters (sozzled can mean drunk).
'alto' with letters rearranged gives 'loat'.
'he' put after 'loat' is 'LOATHE'.
(Other definitions for loathe that I've seen before include "dislike, to a great extent" , "cannot stand" , "Abhor, execrate" , "Strongly detest" , "Feel disgust for (something or someone)" .)