One in bunk causing trouble (4)
I believe the answer is:
riot
'trouble' is the definition.
(thesaurus)
'one in bunk' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'bunk' becomes 'rot' (synonyms).
'i' put inside 'rot' is 'RIOT'.
'causing' is the link.
(Other definitions for riot that I've seen before include "Rampage" , "Uprising" , "Affray, fighting in the streets" , "Debauchery" , "A public affray or disturbance" .)