Because of getting upset over a redhead (3)
I believe the answer is:
for
'because' is the definition.
(thesaurus)
'of getting upset over a redhead' is the wordplay.
'getting upset' shows that the letters should be reversed in order.
'over' says to put letters next to each other.
'a redhead' becomes 'r' (the head/first letter of 'red').
'of' backwards is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "See 24" , "On behalf of" , "In favour of; since" , "and 16 and 20: Notwithstanding" , "On the side of" .)