Paper 'at is required in Poplar? (8)
I believe the answer is:
treatise
'paper' is the definition.
(I've seen this before)
''at is required in poplar?' is the wordplay.
'required in' is an insertion indicator.
'poplar?' becomes 'tree' (poplar is a kind of tree).
'at'+'is'='atis'
'atis' inserted within 'tree' is 'TREATISE'.
(Other definitions for treatise that I've seen before include "Thesis" , "Iterates in dissertation" , "Essay on theme" , "theory" , "Learned work formally dealing with a subject" .)