Secretary in South Africa to return them forthwith (4)
I believe the answer is:
asap
'them forthwith' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'secretary in south africa to return' is the wordplay.
'secretary in' becomes 'pa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'south africa' becomes 'SA' (abbreviation).
'to return' says the letters should be written backwards.
'pa'+'sa'='pasa'
'pasa' in reverse letter order is 'ASAP'.
Can you help me to learn more?
(Other definitions for asap that I've seen before include "Notation for urgent, quick as you can (4)" , "yesterday?" , "Notation meaning without delay, treat as urgent" , "Notation meaning without delay, fast as you can" , "Common acronym for quick-as-you-can" .)