One so lost in quarantine, occupied by routine milk production (9)

'milk production' is the definition.
'lactation' can be an answer for 'milk' (thesaurus). I am unsure of the 'production' bit.

'one so lost in quarantine occupied by routine' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'lost' could be 'l' (abbreviation in sporting league tables) and 'l' is found within the answer.
'by' could be 'at' and 'at' is found within the answer.
'routine' could be 'act' (synonyms) and 'act' is found within the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?