Boss in gumboots (4)

I believe the answer is:

umbo

'boss' is the definition.
(the boss on a shield)

'in gumboots' is the wordplay.
'in' indicates the answer is hidden within the clue.
'UMBO' is hidden within 'gumboots'.

(Another definition for umbo that I've seen is " Boss on a shield".)

I've seen this clue in The Globe and Mail.

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