Chap after small snack (6)
I believe the answer is:
sarnie
'snack' is the definition.
(informal term for a sandwich)
'chap after small' is the wordplay.
'chap' becomes 'arnie' (Arnie is a masculine given name).
'after' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'arnie' put after 's' is 'SARNIE'.
(Other definitions for sarnie that I've seen before include "Sandwich (informal)" , "no classy fare" , "Two slices of bread with a filling" , "Bread and filling snack (informal)" , "Sandwich (slang)" .)