Reservoir to colour agreeably (7)
I believe the answer is:
inkwell
'reservoir' is the definition.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for inkwell that I've seen before include "source for old-fashioned writer" , "Old desktop pot" , "Container sunk unto a desk of old" , "Writing fluid holder" , "Old school desk feature" .)