Son visits American writer in Poplar (5)
I believe the answer is:
aspen
'poplar' is the definition.
(aspen is a kind of poplar)
'son visits american writer' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'visits' is an insertion indicator (visit can mean to go into).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'writer' becomes 'pen' (something which writes).
'a'+'pen'='apen'
's' put within 'apen' is 'ASPEN'.
'in' is the link.
(Other definitions for aspen that I've seen before include "somewhere in Colorado" , "Very timid" , "Trembling poplar tree" , "Trembler" , "Its leaves flutter in the slightest breeze" .)