I don't know secretary's son (4)
I believe the answer is:
pass
'i don't know' is the definition.
(e.g. response in a quiz)
'secretary's son' is the wordplay.
'secretary's' becomes 'PA's' (abbreviation for personal assistant).
'son' becomes 's' (genealogical abbreviation for son).
'pas'+'s'='PASS'
(Other definitions for pass that I've seen before include "Spend (19, say) -- don't know" , "Transfer" , "entry document" , "Overtake - get through" , "Authorise; overtake" .)