Boy to get surgery at last to become a woman (4)
I believe the answer is:
lady
'a woman' is the definition.
(I know that woman can be written as lady)
'boy to get surgery at last' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'to get' says to put letters next to each other.
'at last' suggests the final letters.
The last letter of 'surgery' is 'y'.
'lad'+'y'='LADY'
'to become' acts as a link.
(Other definitions for lady that I've seen before include "See 2" , "High-class woman" , "Lord's wife - peeress's title" , "Gentlewoman" , "Female peer" .)