Everyone entering bumped into hammer (6)
I believe the answer is:
mallet
'hammer' is the definition.
(mallet is a kind of hammer)
'everyone entering bumped into' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'entering' means one lot of letters goes inside another.
'bumped into' becomes 'met' (I've seen this before).
'all' put into 'met' is 'MALLET'.
(Other definitions for mallet that I've seen before include "Kind of hammer used in polo" , "Small wooden hammer" , "Wooden-headed hammer" , "Wooden hammer used in polo" , "It's used in croquet and polo" .)