Programme on small child (5)
I believe the answer is:
sprog
'child' is the definition.
(informal term for a child)
'programme on small' is the wordplay.
'programme' becomes 'prog' (short for programme).
'on' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'prog' put after 's' is 'SPROG'.
(Other definitions for sprog that I've seen before include "Baby (slang)" , "Child, colloquially" , "Kid" , "Child; recruit (mil. slang)" , "Infant" .)