Affair is trivial without first trio (5)
I believe the answer is:
fling
'affair' is the definition.
(a fling is a romantic affair)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fling that I've seen before include "Throw angrily in spell of indulgent pleasure" , "quick one?" , "good time" , "Brief relationship" , "Throw as in Highland dance?" .)