Attachment of a sheet enclosing pound note (9)
I believe the answer is:
appendage
'attachment' is the definition.
(thesaurus)
'a sheet enclosing pound note' is the wordplay.
'sheet' becomes 'page' (page is a kind of sheet).
'enclosing' means one lot of letters goes inside another.
'pound note' becomes 'pend' (I can't explain this - if you can you should give a lot more credence to this answer).
'page' placed around 'pend' is 'ppendage'.
'a'+'ppendage'='APPENDAGE'
'of' acts as a link.
(Other definitions for appendage that I've seen before include "Addendum" , "An outgrowth" , "Thing attached to another" , "Something attached to a larger thing" , "E.g. insect's leg, feeler" .)