What detective may find profits no criminal possessing boot, ultimately (10)
I believe the answer is:
footprints
'what detective may find' is the definition.
(footprints may be a clue in detective stories)
'profits no criminal possessing boot ultimately' is the wordplay.
'criminal' indicates an anagram.
'possessing' is an insertion indicator.
'ultimately' says to take the final letters.
The last letter of 'boot' is 't'.
'profits'+'no'='profitsno'
'profitsno' anagrammed gives 'footprins'.
'footprins' placed around 't' is 'FOOTPRINTS'.
(Other definitions for footprints that I've seen before include "Shoe impressions" , "Steps" .)