Regiments tune in late to second in command (10)

I believe the answer is:

lieutenant

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

'second in command' is the definition.
(thesaurus)

'regiments tune in late' is the wordplay.
'tune in late' can be anagrammed to 'LIEUTENANT'.
But, I'm not clear how the anagram is indicated.

'to' acts as a link.

Can you help me to learn more?

(Other definitions for lieutenant that I've seen before include "Assistant" , "Substitute" , "Army officer" , "Deputy" , "UK rank below captain" .)

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

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