Paint that's aquamarine, say (11)
I believe the answer is:
watercolour
'paint' is the definition.
(I know that paint is a is a more specific form of the action of watercolour)
'that's aquamarine say' is the wordplay.
'that' becomes 'water' (I can't explain this - if you can you should give a lot more credence to this answer).
'aquamarine say' becomes 'colour' (aquamarine is a kind of colour).
'water'+'colour'='WATERCOLOUR'
Can you help me to learn more?
(Other definitions for watercolour that I've seen before include "Type of painting" , "Painted picture" .)