Upset senior has one in tears (5)
I believe the answer is:
races
'tears' is the definition.
(I've seen this in another clue)
'upset senior has one' is the wordplay.
'upset' shows that the letters should be reversed in order.
'senior' becomes 'sr' (abbreviation).
'has' indicates putting letters inside.
'one' becomes 'ace' (ace has a value of one in some card games).
'sr' backwards is 'rs'.
'rs' enclosing 'ace' is 'RACES'.
'in' is the link.
(Other definitions for races that I've seen before include "Speed competitions for various peoples" , "Speed contests for horses, cars etc" , "Competitive trials of speed" , "Scare in sporting contests" , "Sprint and marathon?" .)