We're out to knock what's lacking colour (5)
I believe the answer is:
white
'lacking colour' is the definition.
'white' can be an answer for 'colour' (white is a kind of colour). I am not certain of the 'lacking' bit.
'we're out to knock' is the wordplay.
'we're' becomes 'we' (I can't justify this - if you can you should give a lot more credence to this answer).
'out' indicates putting letters inside.
'to knock' becomes 'hit' ('hit' can be a synonym of 'knock').
'we' enclosing 'hit' is 'WHITE'.
'what's' is the link.
Can you help me to learn more?
(Other definitions for white that I've seen before include "Opponent of six dn (chess)" , "Chess player moving first" , "Outer part of egg" , "Colour of milk" , "Achromatic colour" .)