Shackle letters for protection (9)
I believe the answer is:
chainmail
'for protection' is the definition.
(I have seen 'strong protection' mean 'chainmail' so perhaps 'protection' could also mean 'chainmail')
'shackle letters' is the wordplay.
'shackle' becomes 'chain' (chain is a kind of shackle).
'letters' becomes 'mail' (I've seen this before).
'chain'+'mail'='CHAINMAIL'
(Other definitions for chainmail that I've seen before include "Links designed to protect" , "strong protection" , "protective cover" , "Flexible body armour" .)