Home in tree guarded by blessed good Lord! (8)
I believe the answer is:
honestly
'good lord' is the definition.
(as interjections)
'home in tree guarded by blessed' is the wordplay.
'home in tree' becomes 'nest' (home for birds).
'guarded by' is an insertion indicator.
'blessed' becomes 'holy' (similar in meaning).
'nest' put within 'holy' is 'HONESTLY'.
(Other definitions for honestly that I've seen before include "On the sly (anag.)" , "In a just manner" , "In truth" , "Frankly -- that's outrageous!" , "How law-abiding people act" .)