Part-time deed was planned beforehand (12)

I believe the answer is:

premeditated

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

'planned beforehand' is the definition.
(I've seen this before)

'part-time deed' is the wordplay.
'part-time deed' anagrams to 'PREMEDITATED'.
However, I'm not sure how the anagram is indicated.

'was' is the link.

Can you help me to learn more?

(Other definitions for premeditated that I've seen before include "Planned in advance" , "Planned beforehand" , "Thought out beforehand" , "Considered" .)

I've seen this clue in The Irish News.

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