Criminal let in entrance for a significant anniversary (13)

I believe the answer is:

tercentennial

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

'significant anniversary' is the definition.
'tercentennial' can be an answer for 'anniversary' (tercentennial is a kind of anniversary). I'm not sure about the 'significant' bit.

'criminal let in entrance' is the wordplay.
'criminal' is an anagram indicator.
'let'+'in'+'entrance'='letinentrance'
'letinentrance' is an anagram of 'TERCENTENNIAL'.

'for a' acts as a link.

Can you help me to learn more?

(Another definition for tercentennial that I've seen is " Three-hundredth anniversary".)

I've seen this clue in The Guardian.

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