Trouble started with the next in line of them (5)
I believe the answer is:
their
'of them' is the definition.
(I've seen this before)
'trouble started with the next in line' is the wordplay.
'started with' says to take the initial letters (I've seen 'start with' mean this).
'the next in line' becomes 'heir' (I've seen this before).
The first letter of 'trouble' is 't'.
't'+'heir'='THEIR'
(Other definitions for their that I've seen before include "belonging to an individual" , "Belonging to him (perhaps wrongly)" , "Some would say his (or her)" , "Relating to them" , "Those people's" .)