Clobber boy in front of tree (4)
I believe the answer is:
bash
'clobber' is the definition.
(I've seen this before)
'boy in front of tree' is the wordplay.
'boy' becomes 'b' (abbreviation used in schools).
'in front of' says to put letters next to each other (some letters go in front of others).
'tree' becomes 'ash' (ash is a kind of tree).
'b'+'ash'='BASH'
(Other definitions for bash that I've seen before include "Try - rave-up" , "Smite; a party" , "Heavy blow; party" , "Party (colloq.)" , "Party; beat" .)