Dumbfounded, put on nursing registration number the wrong way round (9)
I believe the answer is:
staggered
'dumbfounded' is the definition.
(staggering is a kind of dumbfounding)
'put on nursing registration number the wrong way round' is the wordplay.
'put on' becomes 'staged' (both can mean fake or affected).
'nursing' means one lot of letters goes inside another.
'registration number' becomes 'reg' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the wrong way round' says the letters should be written backwards.
'reg' reversed gives 'ger'.
'staged' going around 'ger' is 'STAGGERED'.
Can you help me to learn more?
(Other definitions for staggered that I've seen before include "Amazed" , "Arranged to take place at intervals" , "Did floor" , "Deeply shocked or walked unsteadily as if about to fall" , "Lurched, reeled" .)