Giant timber cross etched with name of king, half erased (7)
I believe the answer is:
redwood
'giant timber' is the definition.
(large tree)
'cross etched with name of king half erased' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'etched with' indicates putting letters inside (some letters etch their way into others).
'name of king' becomes 'Edward' (name of several English kings).
'half erased' means to take half the letters.
'edward' halved is 'edw'.
'rood' placed around 'edw' is 'REDWOOD'.
(Other definitions for redwood that I've seen before include "Giant tree of California" , "Tory MP" , "Californian conifer" , "Mad" , "may be growing in California" .)