A writer describing small tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(I know that aspen is a type of poplar)
'a writer describing small' is the wordplay.
'writer' becomes 'pen' (something which writes).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'pen'='apen'
'apen' going around 's' is 'ASPEN'.
(Other definitions for aspen that I've seen before include "One trembling" , "Tree that quivers" , "Trembler" , "Colorado winter sports town" , "winter resort" .)