They steal inoffensive christmas items (12)

I believe the answer is:

safecrackers

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

'they steal' is the definition.
Although both the answer and 'steal' are nouns, they are not in the same form of the noun.

'inoffensive christmas items' is the wordplay.
'inoffensive' becomes 'safe' (I can't explain this - if you can you should believe this answer much more).
'christmas items' becomes 'crackers' (I've seen this before).
'safe'+'crackers'='SAFECRACKERS'

Can you help me to learn more?

I've seen this clue in The Irish Times.

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