Just one line from a letter (5)
I believe the answer is:
serif
'line from a letter' is the definition.
'serif' can be an answer for 'line' (serif is a kind of line). I'm not certain of the 'from a letter' bit.
'just one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for serif that I've seen before include "Letter feature" , "Projection at a letter's end" , "Brazil port" , "Feature of some typefaces" , "Stroke's foot" .)