Harold at first confessed rising up to snap in anger (5)
I believe the answer is:
gnash
'to snap in anger' is the definition.
(as in gnashing one's teeth)
'harold at first confessed rising' is the wordplay.
'at first' says to take the initial letters.
'confessed' becomes 'sang' (to sing can informally mean to confess).
'rising' says the letters should be written in reverse (going up in a down clue).
The first letter of 'harold' is 'h'.
'h'+'sang'='hsang'
'hsang' reversed gives 'GNASH'.
'up' acts as a link.
This may not be correct. It may belong to another bit of the clue.
(Other definitions for gnash that I've seen before include "Grind one's teeth" , "grinding action" , "Grind away" , "Strike one's teeth together in rage" , "champ" .)