Dig; research deeply (5)
I believe the answer is:
delve
I believe this is a double definition.
'dig' is the first definition.
(I know that dig can be written as delve)
'research deeply' is the second definition.
(I've seen this in another clue)
(Other definitions for delve that I've seen before include "Fish around" , "Search deeply for information" , "Dig - research deeply" , "Make painstaking enquiries (into something)" , "Burrow" .)