Writer had upped and left (4)
I believe the answer is:
dahl
'writer' is the definition.
(Roald Dahl)
'had upped and left' is the wordplay.
'upped' indicates anagramming the letters (I've seen 'up' mean this).
'and' says to put letters next to each other.
'left' becomes 'l' (common abbreviation**).
'had' anagrammed gives 'dah'.
'dah'+'l'='DAHL'
(Other definitions for dahl that I've seen before include "'Roald ..., children's author (4)'" , "Roald -, Br. author" , "Split pulses used in Indian cooking" , "British author, notably of children's books, d.1990" , "Roald or Sophie?" .)