Heroin supplier went soft inside, letting head drop (5,4)

I believe the answer is:

poppy seed

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

'heroin supplier went soft' is the definition.
I can't tell whether this defines the answer.

'letting head drop' is the wordplay.
'letting head' becomes 'poppy' (I can't explain this - if you can you should give a lot more credence to this answer).
'drop' becomes 'seed' (I've seen this before).
'poppy'+'seed'='POPPY SEED'

'inside' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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