One tries to cause upset and turmoil - occasionally crossing line (5)
I believe the answer is:
troll
'one tries to cause upset' is the definition.
I can't judge whether this defines the answer.
'turmoil occasionally crossing line' is the wordplay.
'occasionally' means one should take alternating letters.
'crossing' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
The alternate letters of 'turmoil' are 'trol'.
'trol' going around 'l' is 'TROLL'.
'and' is the link.
Can you help me to learn more?
(Other definitions for troll that I've seen before include "Online pest" , "Ugly creature of folklore" , "Fish - cave-elf" , "Fish with a moving bait" , "Cave-dwelling dwarf or giant" .)