Attempt to capture tree’s branching pattern (7)
I believe the answer is:
tracery
'branching pattern' is the definition.
'attempt to capture tree's' is the wordplay.
'attempt' becomes 'try' ('try' can be a synonym of 'attempt').
'to capture' indicates putting letters inside.
'tree' becomes 'acer' (I've seen this before).
'try' placed around 'acer' is 'TRACERY'.
(Other definitions for tracery that I've seen before include "Delicate ornamental stonework" , "Stone work in Gothic windows" , "Thin ribs of e.g. Gothic window" , "Gothic openwork" , "Delicate stone work" .)