Scrap dealer's last in garden building (5)
I believe the answer is:
shred
'scrap' is the definition.
(both can mean a small amount of material)
'dealer's last in garden building' is the wordplay.
'last' says to take the final letters.
'in' means one lot of letters goes inside another.
'garden building' becomes 'shed' (I've seen this in another clue).
The final letter of 'dealer' is 'r'.
'r' inserted within 'shed' is 'SHRED'.
(Other definitions for shred that I've seen before include "Rip up" , "Small part" , "Scrap of material" , "Bit of cloth" , "Cut up; a small amount" .)