Linesmen who lament get less upset, I admitted (8)
I believe the answer is:
elegists
'linesmen who lament' is the definition.
I can't judge whether this definition defines the answer.
'get less upset i admitted' is the wordplay.
'upset' indicates anagramming the letters.
'admitted' indicates putting letters inside (inserted letters get admitted).
'get'+'less'='getless'
'getless' is an anagram of 'elegsts'.
'elegsts' enclosing 'i' is 'ELEGISTS'.
Can you help me to learn more?
(Other definitions for elegists that I've seen before include "pensive poems penned by us?" , "Poets of a kind" .)