Crossword Genius

Lager available for vulnerable child (6,3)

Ross

I believe the answer is:

foster son

I'm a little stuck... Click here to teach me more about this clue!

'child' is the definition.
(I know that foster-son is a type of foster child)

'lager available for vulnerable' is the wordplay.
I cannot quite see how this works, but
'available' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
The remaining letters 'fosters' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for foster son that I've seen is " re-housed kid?".)

I've seen this clue in The Independent.
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