Contest well-matched over time (5)
I believe the answer is:
event
'contest' is the definition.
(thesaurus)
'well-matched over time' is the wordplay.
'well-matched' becomes 'even'.
'over' means one lot of letters go next to another.
'time' becomes 't'.
'even'+'t'='EVENT'
(Other definitions for event that I've seen before include "Item in sports programme" , "'Occurrence, occasion (5)'" , "400 metres?" , "High jump, say" , "What will happen" .)