As it were, number two of teachers filled in, head being nauseous (10)

'as' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'it were number two of teachers filled in head being nauseous' is the wordplay.
I cannot really see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'number' could be 'i' (I is an example) and 'i' is present in the answer.
'head' could be 'ness' (ness is a headland) and 'ness' is found in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?