Author has upset boy penning a bit of horror (4)
I believe the answer is:
dahl
'author' is the definition.
(I've seen this before)
'upset boy penning a bit of horror' is the wordplay.
'upset' shows that the letters should be reversed in order.
'boy' becomes 'lad' (informal term for boy).
'penning' is an insertion indicator.
'a bit of' says to take the initial letters.
The first letter of 'horror' is 'h'.
'lad' going around 'h' is 'lhad'.
'lhad' in reverse letter order is 'DAHL'.
'has' acts as a link.
(Other definitions for dahl that I've seen before include "Roald -, British writer" , "Willy Wonka's creator" , "British writer of many children's books, d. 1990" , "British writer notably of children's books" , "Roald or Sophie?" .)