Convict bottom has to drag chain (3,6)

I believe the answer is:

lag behind

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'drag chain' is the definition.
'lag behind' can be an answer for 'drag' (thesaurus). I am not sure about the 'chain' bit.

'convict bottom' is the wordplay.
'convict' becomes 'lag' (slang term for a prisoner).
'bottom' becomes 'behind' (synonyms).
'lag'+'behind'='LAG BEHIND'

'has to' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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