Svelte Timothy runs inside (4)
I believe the answer is:
trim
'svelte' is the definition.
(I've seen this in another clue)
'timothy runs inside' is the wordplay.
'timothy' becomes 'tim' (I've seen this in other clues).
'runs' becomes 'r' (cricket abbreviation).
'inside' indicates putting letters inside.
'tim' enclosing 'r' is 'TRIM'.
(Other definitions for trim that I've seen before include "Make neat; slim and fit" , "Dress - prune" , "fittings" , "Well-kept - tidy" , "Light hair-cut in Meath" .)