Marriage, an equal contest that is likely to ignite (5)
I believe the answer is:
match
This might be a double definition.
'marriage' is the first definition.
(I've seen this before)
'an equal contest that is likely to ignite' is the second definition.
'match' can be an answer for 'an equal' ('match' can be a synonym of 'equal'). I am unsure of the rest of the definition.
Can you help me to learn more?
(Other definitions for match that I've seen before include "Suit; fixture" , "Sporting event" , "Correspondence" , "Alliance" , "Lighter; contest" .)