Have been in bed; was so good! (7)

I believe the answer is:

behaved

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

'bed was so good' is the definition.
I can't tell whether this defines the answer.

'have been in' is the wordplay.
'been' becomes 'bed' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'have' inserted inside 'bed' is 'BEHAVED'.

Can you help me to learn more?

(Another definition for behaved that I've seen is " Conducted oneself in a certain manner".)

I've seen this clue in the Sydney Morning Herald.

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