Alarm about the light thing (7)
I believe the answer is:
feather
'light thing' is the definition.
'feather' can be an answer for 'thing' (I have seen 'trifling thing' mean 'feather' so perhaps 'thing' could also mean 'feather'). I am not certain of the 'light' bit.
'alarm about the' is the wordplay.
'alarm' becomes 'fear' (I've seen this before).
'about' indicates putting letters inside.
'fear' enclosing 'the' is 'FEATHER'.
Can you help me to learn more?
(Other definitions for feather that I've seen before include "English plume (de ma tante?)" , "In your cap, this represents an achievement" , "Wing component" , "thing of little importance" , "Prov. light part of bird" .)