Detectives run from ruthless criminal (7)
I believe the answer is:
'detectives' is the definition.
(sleuth is a kind of detective)
'run from ruthless criminal' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'from' is a deletion indicator.
'criminal' is an anagram indicator.
'ruthless' with 'r' taken out is 'uthless'.
'uthless' anagrammed gives 'SLEUTHS'.
(Other definitions for sleuths that I've seen before include "Detectives" , "Private eyes" .)