Rule room number's to be put first (4)
I believe the answer is:
norm
'rule' is the definition.
(norm is a kind of rule)
'room number's to be put first' is the wordplay.
'room' becomes 'rm' (abbreviation for room).
'number' becomes 'no' (abbreviation for 'number').
'to be put first' says to put letters next to each other.
'rm' after 'no' is 'NORM'.
(Other definitions for norm that I've seen before include "Expected standard" , "Average level of achievement" , "Yardstick" , "expected thing" , "Average level of performance" .)