Writer has to decide colour (3,5)
I believe the answer is:
pea green
'colour' is the definition.
(I know this)
'writer has to decide' is the wordplay.
'writer' becomes 'pen' (something which writes).
'has' means one lot of letters goes inside another.
'to decide' becomes 'eagre' (I can't explain this - if you can you should give a lot more credence to this answer).
'pen' enclosing 'eagre' is 'PEA-GREEN'.
Can you help me to learn more?
(Other definitions for pea green that I've seen before include "Colour of Owl and Pussycat's boat (Lear)" , "colour of Lear's boat" , "Bright colour (of vegetables)" , "Shade of the Owl and the Pussycat's boat" , "Vegetable shade?" .)