A small fear - could be an unnecessary alert (5,5)
I believe the answer is:
false alarm
'an unnecessary alert' is the definition.
The definition suggests a singular noun which matches the answer.
'a small fear could be' is the wordplay.
'could be' indicates an anagram.
'a'+'small'+'fear'='asmallfear'
'asmallfear' is an anagram of 'FALSE ALARM'.
Can you help me to learn more?
(Other definitions for false alarm that I've seen before include "unfounded scare?" , "Warning proving unnecessary" , "Panic over" .)