A left limb anxiously raised? (5)

I believe the answer is:

alarm

I'm a little stuck... Click here to teach me more about this clue!

'a left limb anxiously raised?' is the definition.
I can't tell whether this defines the answer.

'a left limb' is the wordplay.
'a left' becomes 'al' (I've seen this in other clues).
'limb' becomes 'arm' (arm is a kind of limb).
'al'+'arm'='ALARM'

Can you help me to learn more?

(Other definitions for alarm that I've seen before include "Agitate" , "Feeling of fear" , "Frighten, warning" , "Burglar deterrent" , "Device sounding danger" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app