Handed trainee a mallet (5)
I believe the answer is:
gavel
'a mallet' is the definition.
(gavel is a kind of mallet)
'handed trainee' is the wordplay.
'handed' becomes 'gave' ('give' can be a synonym of 'hand').
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'gave'+'l'='GAVEL'
(Other definitions for gavel that I've seen before include "Ceremonial mallet" , "Auctioneer's mallet" , "Judicial hammer" , "Small hammer" , "Mason's hammer" .)