When to fence in tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'when to fence' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'to fence' becomes 'pen' (I can't explain this - if you can you should give a lot more credence to this answer).
'as'+'pen'='ASPEN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for aspen that I've seen before include "Tree, kind of poplar" , "Panes (anag.)" , "Its leaves flutter in the slightest breeze" , "Tremulous poplar" , "somewhere in Colorado" .)