Describing every detail, expose illegitimate child (4-2-4)
I believe the answer is:
blow-by-blow
'describing every detail' is the definition.
(as in a blow-by-blow account)
'expose illegitimate child' is the wordplay.
'expose' becomes 'blow' (as in blowing someone's cover).
'illegitimate child' becomes 'byblow' (by-blow can mean an illegitimate child).
'blow'+'byblow'='BLOW-BY-BLOW'
(Other definitions for blow-by-blow that I've seen before include "Comprehensive" , "Recounted in (whopping?) detail" , "Giving all details as they happened" , "boxing?" , "Explained in great detail" .)