Fathead much upset over a poor start (5)
I believe the answer is:
chump
'fathead' is the definition.
(thesaurus)
'much upset over a poor start' is the wordplay.
'upset' is an anagram indicator.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'start' says to take the initial letters.
The first letter of 'poor' is 'p'.
'much' is an anagram of 'chum'.
'chum'+'p'='CHUMP'
(Other definitions for chump that I've seen before
include
"Foolish person"
,
"Nitwit"
,
"Dolt"
,
"Thick