One's kept in the shade - that's a troublesome thing (8)
I believe the answer is:
nuisance
'a troublesome thing' is the definition.
(I've seen this before)
'one's kept in the shade' is the wordplay.
'one's' becomes 'is' (roman numeral).
'kept in' indicates putting letters inside.
'the shade' becomes 'nuance' ('nuance' can be a synonym of 'shade'**).
'is' put into 'nuance' is 'NUISANCE'.
'that's' is the link.
(Other definitions for nuisance that I've seen before include "Annoying thing/person" , "Source of annoyance or aggravation" , "Source of irritation, person or thing" , "Bothersome pest" , "Something which annoys" .)