Thief in small room starts to take everything regardless (6)
I believe the answer is:
looter
'thief' is the definition.
(looter is a kind of thief)
'small room starts to take everything regardless' is the wordplay.
'small room' becomes 'loo'.
'starts to' says to take the initial letters.
The initial letters of 'take everything regardless' is 'ter'.
'loo'+'ter'='LOOTER'
'in' acts as a link.
(Other definitions for looter that I've seen before include "robber" , "Stealer of goods" , "Wartime thief" , "Wartime stealer" , "Thief during unrest" .)