Device for fastening plaster cast (7)
I believe the answer is:
stapler
'device' is the definition.
(stapler is a kind of device)
'plaster cast' is the wordplay.
'cast' indicates an anagram (letters are dispersed or cast around).
'plaster' anagrammed gives 'STAPLER'.
'for fastening' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stapler that I've seen before include "Implement for inserting paper fasteners" , "Office equipment - plaster (anag)" , "Office fastener" , "Item of office equipment" , "Paper-fastening device" .)