Children’s author had to come up with line (4)
I believe the answer is:
dahl
'children's author' is the definition.
(Roald Dahl)
'had to come up with line' is the wordplay.
'to come up' shows that the letters should be reversed in order.
'with' says to put letters next to each other.
'line' becomes 'l' (used when specifying particular lines from a poem).
'had' in reverse letter order is 'dah'.
'dah'+'l'='DAHL'
(Other definitions for dahl that I've seen before include "Roald -, Br. author" , "British writer notably of children's books" , "Model" , "British writer of many children's books, d. 1990" , "Roald -, British writer" .)