Express anger when an oaf circles a tree (4,3)
I believe the answer is:
lash out
'express anger' is the definition.
(to lash out at someone is to get angry with them)
'an oaf circles a tree' is the wordplay.
'an oaf' becomes 'lout' ('lout' can be a synonym of 'oaf').
'circles' means one lot of letters goes inside another.
'a tree' becomes 'ash' (ash is a kind of tree).
'lout' enclosing 'ash' is 'LASH OUT'.
'when' acts as a link.
(Other definitions for lash out that I've seen before include "React aggressively" , "Wildly attack" , "Become suddenly aggressive" , "Spend lots" , "unprovoked attack" .)