To be a cadger is an absorbing thing (6)

I believe the answer is:

sponge

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

'absorbing thing' is the definition.
'sponge' can be an answer for 'absorbing' (I've seen this before). I'm not sure about the 'thing' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sponge that I've seen before include "Aquatic invertebrate -- type of cake" , "Porous fibres for soaking up" , "Freeload" , "Soft aquatic creature" , "Absorbent cleaner - cadge" .)

I've seen this clue in the Evening Standard.

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