Extremely useful bird boxes at last! (8)
I believe the answer is:
ultimate
'last' is the definition.
(similar in meaning)
'extremely useful bird boxes at' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'bird' becomes 'time' (I've seen this before).
'boxes' is an insertion indicator (the outside letters must 'box' inside letters in).
'useful' with its centre removed is 'ul'.
'time' enclosing 'at' is 'timate'.
'ul'+'timate'='ULTIMATE'
(Other definitions for ultimate that I've seen before include "Final or absolute" , "Eventual" , "Mutilate in last possible way" , "Utmost or extreme" , "Furthest, final" .)