One time alarm goes off and it's unimportant! (10)
I believe the answer is:
immaterial
'it's unimportant' is the definition.
(I've seen this before)
'one time alarm goes off' is the wordplay.
'one' becomes 'i' (Roman numeral).
'goes off' is an anagram indicator.
'time'+'alarm'='timealarm'
'timealarm' anagrammed gives 'mmaterial'.
'i'+'mmaterial'='IMMATERIAL'
'and' acts as a link.
(Other definitions for immaterial that I've seen before include "of no importance" , "unimportant" , "Irrelevant" .)