Looked at one way, 2020 was first of two parts (6-9)

'looked at one' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally define each other.

'way 2020 was first of two parts' is the wordplay.
I cannot quite understand how this works, but
'way' could be 'barrel' (barrel is a kind of way) and 'barrel' is found within the answer.
'first' could be 'double' (double first is a kind of first) and 'double' is found in the answer.
The remaining letters 'led' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?