In pain, did stagger when walking (7)

I believe the answer is:

padding

'walking' is the definition.
(padding is a kind of walking)

'in pain did stagger' is the wordplay.
'in' indicates putting letters inside.
'pain' becomes 'pang' (pang is a kind of pain).
'stagger' indicates anagramming the letters (I've seen 'staggering' mean this (letters stagger around)).
'did' with letters rearranged gives 'ddi'.
'pang' placed around 'ddi' is 'PADDING'.

'when' acts as a link.

(Another definition for padding that I've seen is " Soft stuffing".)

I've seen this clue in the Evening Standard.

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