Writer outside around tree (5)
I believe the answer is:
pecan
'tree' is the definition.
(I know that pecan is a type of nut tree)
'writer outside around' is the wordplay.
'writer' becomes 'pen' (something which writes).
'outside' means one lot of letters goes inside another.
'around' becomes 'ca' (circa).
'pen' placed around 'ca' is 'PECAN'.
(Other definitions for pecan that I've seen before include "Smooth pinkish-brown nut, nice in pie" , "US nut tree" , "Walnut-like nut" , "Nutty type" , "of nuts" .)