Offspring is a girl (5)
I believe the answer is:
issue
'offspring' is the definition.
(I know that offspring can be written as issue)
'is a girl' is the wordplay.
'a girl' becomes 'sue' (girl's name).
'is'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "Sprogs" , "important subject" , "Offspring, or the point in question" , "Topic (to be addressed)" , "Difficulty" .)