Donated first lieutenant's hammer (5)
I believe the answer is:
gavel
'hammer' is the definition.
(I know that gavel is a type of hammer)
'donated first lieutenant's' is the wordplay.
'donated' becomes 'gave' (give can mean to donate).
'first lieutenant's' becomes 'l' (1st letter of 'lieutenant').
'gave'+'l'='GAVEL'
(Other definitions for gavel that I've seen before include "Ceremonial mallet" , "Judicial hammer" , "Chairman's hammer" , "Judge's hammer" , "equipment for auctioneer" .)