A line written about singer for sport (7)
I believe the answer is:
archery
'sport' is the definition.
(archery is a kind of sport)
'a line written about singer' is the wordplay.
'line' becomes 'ry' (abbreviation for railway).
'written about' indicates putting letters inside.
'singer' becomes 'cher' (I've seen this in other clues).
'ry' going around 'cher' is 'rchery'.
'a'+'rchery'='ARCHERY'
'for' is the link.
(Other definitions for archery that I've seen before include "Bowman's sport" , "The sport of bowmen" , "Bowmanship" , "Toxophily" , "Bow and arrow sport" .)