A structure, that is, in outer Poplar (4)
I believe the answer is:
pier
'a structure' is the definition.
(pier is a kind of structure)
'that is in outer poplar' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'in' means one lot of letters goes inside another.
'outer' suggests removing the centre.
'poplar' with its middle taken out is 'pr'.
'ie' put inside 'pr' is 'PIER'.
(Other definitions for pier that I've seen before include "Seaside attraction" , "Surviving antiquity" , "Promenade over water" , "Part of airport terminal; breakwater" , "Platform out to sea" .)