Black line last to turn a different colour (5)
I believe the answer is:
brown
'a different colour' is the definition.
'brown' can be an answer for 'colour' (brown is a colour). I am not sure about the 'a different' bit.
'black line last to turn' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil hardness).
'line' becomes 'row' (a row or line of things).
'last to' suggests the final letters.
The final letter of 'turn' is 'n'.
'b'+'row'+'n'='BROWN'
Can you help me to learn more?
(Other definitions for brown that I've seen before include "Dark - tanned - successor to Blair?" , "bay" , "Tan" , "(Of a site) previously built on" , "Rugby hero" .)