Dicky may need one when replacing dull actors (6,4)

I believe the answer is:

collar stud

'dicky may need one' is the definition.
(a dicky bow tie may need one)

'replacing dull actors' is the subsidiary indication.
'replacing' indicates an anagram (placing again).
'dull'+'actors'='dullactors'
'dullactors' with letters rearranged gives 'collarstud'.

'when' is the link.

I've seen this clue in The Guardian.

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