One of Boris's progeny? (5)
I believe the answer is:
sprog
'one' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'of boris's progeny?' is the wordplay.
'of' indicates a hidden word.
'SPROG' is hidden within 'boris progeny'.
(Other definitions for sprog that I've seen before include "Kid" , "Infant" , "Child, colloquially" , "Child; recruit (mil. slang)" , "Baby (slang)" .)