Second sport makes us curse (5)
I believe the answer is:
swear
'curse' is the definition.
(I know that curse can be written as swear)
'second sport' is the wordplay.
'second' becomes 's' (common abbreviation).
'sport' becomes 'wear' (sport can mean to wear or display on one's person).
's'+'wear'='SWEAR'
'makes us' is the link.
(Other definitions for swear that I've seen before include "Assert" , "Emit oath or take vow" , "Eff and blind" , "Utter obscenities or take an oath" , "Use bad language" .)