Children’s writer Liberal had taken up earlier (4)
I believe the answer is:
dahl
'children's writer' is the definition.
(I've seen this before)
'liberal had taken up earlier' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'taken up' says the letters should be written in reverse.
'earlier' is a charade indicator (letters next to each other).
'had' back-to-front is 'dah'.
'l' put after 'dah' is 'DAHL'.
(Other definitions for dahl that I've seen before include "Indian dish of legumes and spices" , "Willy Wonka's creator" , "Model" , "Roald or Sophie?" , "British author, notably of children's books, d.1990" .)