4 down
One
colour
only
–
second
one
not
included
(8)
One colour only – second one not included (8)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
The definition is One colour only. The wordplay builds the answer in parts: MO comes from "second" (as in "hang on a mo"), then NOT is taken from the clue word "not", and finally ONE comes from the clue word "one". The word "included" signals that NOT sits inside the construction, but really it's a straightforward charade: MO + NOT + ONE = MONOTONE.