Hammer everyone encountered outside (6)
I believe the answer is:
mallet
'hammer' is the definition.
(I know that mallet is a type of hammer)
'everyone encountered outside' is the wordplay.
'everyone' becomes 'all' (all people).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'outside' means one lot of letters goes inside another.
'all' going within 'met' is 'MALLET'.
(Other definitions for mallet that I've seen before include "It's used in croquet and polo" , "Wooden-headed hammer" , "Used to beat" , "An auctioneer's wooden hammer" , "Kind of hammer used in polo" .)