Part of a foot kept by Darwin (Charles) (4)
I believe the answer is:
inch
'part of a foot' is the definition.
('inch' is part of 'foot')
'kept by darwin charles' is the wordplay.
'kept by' says the answer is hidden in the clue.
'INCH' is hidden in the letters of 'darwin charles'.
(Other definitions for inch that I've seen before include "Short length" , "Move by small amounts" , "2.45 cm" , "0.0254 metre" , "Move slowly, in small steps" .)