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' indicates putting letters inside.
'registration number' becomes 'reg' (I can't explain this - if you can you should give a lot more credence to this answer).
'the wrong way round' shows that the letters should be reversed in order.
'reg' back-to-front is 'ger'.
'staged' enclosing '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" .)