He lacks resolve for prisoner's restraint (7)

I believe the answer is:

shackle

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

'prisoner's restraint' is the definition.
'shackle' can be an answer for 'restraint' (shackle is a kind of restraint). I'm unsure of the 'prisoner's' bit.

'he lacks resolve' is the wordplay.
'resolve' is an anagram indicator (I've seen 'resolving' mean this).
'he'+'lacks'='helacks'
'helacks' anagrammed gives 'SHACKLE'.

'for' is the link.

Can you help me to learn more?

(Other definitions for shackle that I've seen before include "Pair of fetters" , "Restrict" , "Chain with fetters" , "Chain, limit" , "Manacle" .)

I've seen this clue in The Daily Express.

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