As one actor appears occasionally – the boy's four, see (10)

'as' is the definition.
Both the definition and answer are adverbs.
Maybe they are linked in a way I don't understand?

'one actor appears occasionally the boy's four see' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'see' could be 'ely' (see is a place with a cathedral - e.g. Ely) and 'ely' is found in the answer.
'the boy's' could be 'he' (he refers to a man or boy) and 'he' is found in the answer.
'boy' could be 'cos' and 'cos' is found in the remaining letters.
A single letter 'v' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?