Mischievous line about European sport (7)
I believe the answer is:
archery
'sport' is the definition.
(archery is a kind of sport)
'mischievous line about european' is the wordplay.
'mischievous' becomes 'arch' (arch can mean mischievous or playful).
'line' becomes 'ry' (abbreviation for railway).
'about' is a charade indicator (letters next to each other).
'european' becomes 'E' (abbreviation e.g. EU).
'ry' after 'e' is 'ery'.
'arch'+'ery'='ARCHERY'
(Other definitions for archery that I've seen before include "Bowmanship" , "The sport of shooting arrows with a bow" , "Arrow-shooting" , "The sport of bowmen" , "Sport of the bowman" .)