Noblewoman unexpectedly bears son (8)
I believe the answer is:
baroness
'noblewoman' is the definition.
(baroness is a kind of noblewoman)
'unexpectedly bears son' is the wordplay.
'unexpectedly' indicates an anagram.
'bears'+'son'='bearsson'
'bearsson' is an anagram of 'BARONESS'.
(Other definitions for baroness that I've seen before include "Thatcher possibly" , "Female life peer" , "Woman life-peer" , "Noblewoman" , "Lady peer" .)