Writer has to run off with this girl (8)
I believe the answer is:
penelope
'this girl' is the definition.
'writer has to run off' is the wordplay.
'writer' becomes 'pen' (something which writes).
'has' means one lot of letters go next to another.
'to run off' becomes 'elope' ('elope' can be a synonym of 'run off').
'pen'+'elope'='PENELOPE'
'with' acts as a link.
(Other definitions for penelope that I've seen before include "Odysseus's wife" , "Wife of Odysseus or Ulysses" , "Mrs Odysseus" , "Woman" , "lady" .)