Child's yellow — doctors gathering round (5)
I believe the answer is:
'child's' is the definition.
(informal term for a child)
'yellow doctors gathering round' is the wordplay.
'yellow' becomes 'or' (heraldic term).
'doctors' becomes 'gps' (GP is an example).
'gathering' indicates putting letters inside (gather can mean to acquire or bring in).
'round' is a reversal indicator.
'or' going within 'gps' is 'gorps'.
'gorps' reversed gives 'SPROG'.
(Other definitions for sprog that I've seen before include "Child, colloquially" , "Child; recruit (mil. slang)" , "little one" , "Infant" , "Kid" .)