Shes always on the line (3)
I believe the answer is:
peg
'the line' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for peg that I've seen before include "Kind of clip for fastening clothes to line" , "Short pin or bolt" , "that may have hat or coat on" , "Old song: ''... o' my heart''" , "Projecting pin" .)