Sleuth Mike fills gaps (6)
I believe the answer is:
holmes
'sleuth' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'mike fills gaps' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'fills' means one lot of letters goes inside another.
'gaps' becomes 'holes' (hole is a kind of gap).
'm' placed into 'holes' is 'HOLMES'.
Can you help me to learn more?
(Other definitions for holmes that I've seen before include "One good at interpreting clues" , "Two-hundred-and-twenty-one-b Baker Street resident" , "Our 1/4 Across [GOLD MEDALLIST] in 2004" , "Sherlock . . . . . . is a great fictional detective" , "Baker Street sleuth" .)