Hairpiece in reserve for sport (5)
I believe the answer is:
rugby
'sport' is the definition.
(rugby is a kind of sport)
'hairpiece in reserve' is the wordplay.
'hairpiece' becomes 'rug' (rug is a kind of hairpiece).
'in reserve' becomes 'by' ('by' can be a synonym of 'in reserve').
'rug'+'by'='RUGBY'
'for' is the link.
(Other definitions for rugby that I've seen before include "Fifteen-a-side sport -- Olympic event in 1900, 1908, 1920 and 1924" , "Sport with oval ball" , "Try game?" , "Midlands town" , "Type of football; school" .)