Investigate thoroughly (5)

I think it could be either:

Which length description are you looking for?:

3,4

delve

(Other definitions for delve that I've seen before include "Look closely (into)" , "Dig - investigate" , "Intensively research (into something)" , "Fish around" , "Reach down and search" .)

I've seen this clue in the Sydney Morning Herald and the Newsday.

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