Line not seen in tree's curve (4)
I believe the answer is:
arch
'curve' is the definition.
(synonyms)
'line not seen in tree's' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'not seen in' is a deletion indicator.
'tree' becomes 'larch' (larch is a kind of tree).
'larch' with 'l' taken away is 'ARCH'.
(Other definitions for arch that I've seen before include "Foremost" , "Curve of a bridge" , "Embow" , "Devilish" , "This could be Wellington" .)