Trembling poplar in Anglo-Saxon enclosure (5)
I believe the answer is:
aspen
'trembling poplar' is the definition.
(I've seen this before)
'anglo-saxon enclosure' is the wordplay.
'anglo-saxon' becomes 'AS' (abbreviation for Anglo-Saxon).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'as'+'pen'='ASPEN'
'in' acts as a link.
(Other definitions for aspen that I've seen before include "Its leaves flutter in the slightest breeze" , "city in Colorado" , "Tree that quivers" , "somewhere in Colorado" , "Trembler" .)