Was anchor commoner before, when overrun by fish? (8)

'overrun by fish?' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives occasionally define each other.

'was anchor commoner before' is the wordplay.
I cannot quite understand how this works, but
'before' could be 'ere' (usually literary) and 'ere' is located in the answer.
This may be the basis of clue (or it may be nonsense).

'when' acts as a link.

Can you help me to learn more?