Loveliest birdhouse had one copperhead sneak in (6)
I believe the answer is:
nicest
'loveliest birdhouse had' is the definition.
I can't judge whether this defines the answer.
'one copperhead sneak in' is the wordplay.
'one' becomes 'i' (Roman numeral).
'copperhead' becomes 'c' (head letter of 'copper').
'sneak' becomes 'nest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'i'+'c'='ic'
'ic' put into 'nest' is 'NICEST'.
Can you help me to learn more?
(Other definitions for nicest that I've seen before include "Insect (anag)" , "Most acceptable" , "Most pleasing" , "Best" , "I scent in the most pleasant way" .)