1971 comedy-crime film starring Albert Finney and Billie Whitelaw (7)

I believe the answer is:

gumshoe

(Other definitions for gumshoe that I've seen before include "One like Sam Spade" , "Detective (US colloq.)" , "Foreign investigator" , "Galosh - sleuth" , "US detective (informal)" .)

I've seen this clue in The Mirror.

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