Slender man runs inside (4)
I believe the answer is:
trim
'slender' is the definition.
(I've seen this in another clue)
'man runs inside' is the wordplay.
'man' becomes 'tim' (short for Timothy).
'runs' becomes 'r' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'tim' enclosing 'r' is 'TRIM'.
(Other definitions for trim that I've seen before include "Neaten by cutting" , "Reduce slightly" , "Neat and tidy as a town in Meath" , "Neat - prune" , "Make smooth by cutting away irregular parts" .)