Worker is first to undertake a number of assignments historically setting people free (12)

'assignments historically setting people free' is the definition.
I can't judge whether this defines the answer.

'worker is first to undertake a number' is the wordplay.
'worker' becomes 'man' (man is a kind of worker).
'is first to' says to take the initial letters.
'a number' becomes 'missions' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'undertake' is 'u'.
'man'+'u'+'missions'='MANUMISSIONS'

'of' acts as a link.

Can you help me to learn more?