Irrelevant alarm I time to go off (10)
I believe the answer is:
immaterial
'irrelevant' is the definition.
(similar in meaning)
'alarm i time to go off' is the wordplay.
'to go off' indicates an anagram.
'alarm'+'i'+'time'='alarmitime'
'alarmitime' anagrammed gives 'IMMATERIAL'.
(Other definitions for immaterial that I've seen before include "of no importance" , "unimportant" , "Irrelevant" .)