Happening to have spotted appropriating a number (9)
I believe the answer is:
seventeen
'number' is the definition.
(seventeen is a kind of whole number)
'happening to have spotted appropriating a' is the wordplay.
I cannot really see how this works, but
'happening' could be 'even' (I've seen this in another clue) and 'even' is found in the answer.
The remaining letters 'steen' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for seventeen that I've seen before include "Figure" , "XVII" , "odd number" , "next clue" , "Cardinal number" .)