In functional terms, 'when it was time for tea' has the same role as
'at tea time' in 'John came round at tea time'. It specifies the time
at which John came round, and we can easily substitute the one for the
other without changing the overall structure of the sentence. So 'when
it was time for tea' is an Adverbial. But it is clearly a clause as well
(it has a Predicator 'was' and a SPOCA structure -Cj SPCA), one which
is nested inside the main clause of the sentence. Because it is a nested
clause which acts as an Adverbial, we call it an Adverbial Clause. We
can draw a tree diagram for it as follows: