Tree line with curved form (5)
I believe the answer is:
larch
'tree' is the definition.
(larch is a kind of tree)
'line with curved form' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'with' says to put letters next to each other.
'curved form' becomes 'arch' (I've seen this before).
'l'+'arch'='LARCH'
(Other definitions for larch that I've seen before include "Deciduous conifer" , "Coniferous tree with deciduous bright green needles" , "Supplier of timber" , "wood" .)