Aid for delivery rider under duress (7)
I believe the answer is:
forceps
'aid for delivery' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'rider under duress' is the wordplay.
'rider' becomes 'PS' (a rider is a clause added to a contract, so a P.S.).
'under' means one lot of letters go next to another (below, in a down clue).
'duress' becomes 'force' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ps' put after 'force' is 'FORCEPS'.
Can you help me to learn more?
(Other definitions for forceps that I've seen before include "Surgical implement" , "surgical equipment" , "may assist delivery" , "Pair of surgical pincers" , "Used by surgeon" .)
