I heard it’s black line — one on face (7)
I believe the answer is:
eyebrow
'one on face' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'i heard it's black line' is the wordplay.
'heard' shows a homophone (sound like).
'it's black' becomes 'b' (as in pencil classifications).
'line' becomes 'row' (row is a kind of line).
'i' sounds like 'eye'.
'eye'+'b'+'row'='EYEBROW'
Can you help me to learn more?
(Other definitions for eyebrow that I've seen before include "It's lifted in surprise" , "this might be raised in surprise" , "something going up doubtfully" , "It may be raised" , "Arch of hair" .)