Last year's pen (3)
I believe the answer is:
sty
'last year's pen' is the definition.
'sty' can be an answer for 'pen' (both can mean an animal enclosure). I am unsure of the 'last year's' bit.
'last year's pen' is the wordplay.
'pen' says the answer is hidden in the clue (I've seen 'pens' mean this (pen can mean to contain or cage)).
'STY' is hidden within 'last years'.
Can you help me to learn more?
(Other definitions for sty that I've seen before include "Pigs' pen" , "Porcine pen" , "Pig's enclosure" , "Piggery" , "Pig house" .)