Hammer donated to head of library (5)
I believe the answer is:
gavel
'hammer' is the definition.
(gavel is a kind of hammer)
'donated to head of library' is the wordplay.
'donated' becomes 'gave' (give can mean to donate).
'to head of' suggests taking the first letters.
The initial letter of 'library' is 'l'.
'gave'+'l'='GAVEL'
(Other definitions for gavel that I've seen before include "Judicial hammer" , "Auctioneer's mallet" , "Mason's hammer" , "Judge's hammer" , "equipment for auctioneer" .)