Rumour first group is out of time (5)
I believe the answer is:
noise
'rumour' is the definition.
(I've seen this in other clues)
'first group is out of time' is the wordplay.
'first' becomes 'noi' (resembles No. 1).
'group' becomes 'set' (both can mean a collection).
'is out of' is a deletion indicator (to be out of something is to no longer have it).
'time' becomes 't' (abbreviation).
'set' with 't' taken out is 'se'.
'noi'+'se'='NOISE'
(Other definitions for noise that I've seen before include "Confused sounds" , "Clamour" , "Kind of pollution" , "Loud or disturbing sound" , "Clatter" .)