Writer is about to approve a colour (3,5)
I believe the answer is:
pea green
'a colour' is the definition.
(pea green is a kind of colour)
'writer is about to approve' is the wordplay.
'writer' becomes 'pen' (something which writes).
'is about' is an insertion indicator.
'to approve' becomes 'agree' (I can't justify this - if you can you should believe this answer much more).
'pen' enclosing 'agree' is 'PEA-GREEN'.
Can you help me to learn more?
(Other definitions for pea green that I've seen before include "colour of Lear's boat" , "The colour of podded vegetables and the Owl and the Pussycat's boat" , "Bright colour with a yellow tinge - eager pen (anag)" , "Bright vegetable colour" , "Shade of the Owl and the Pussycat's boat" .)