Injure hot member in a cast? (4)

I believe the answer is:

harm

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'injure' is the definition.
(I know that harm is a more specific form of injure)

'hot member in a cast?' is the wordplay.
'hot' becomes 'h' (abbreviation, eg on taps).
'member in a cast?' becomes 'arm' (I can't explain this - if you can you should give a lot more credence to this answer).
'h'+'arm'='HARM'

Can you help me to learn more?

(Other definitions for harm that I've seen before include "Injure or damage" , "Hurt, maim" , "Short-long metric feet" , "Damage, hurt" , "Injure, injury" .)

I've seen this clue in The Times.

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