Animal pen at end of lane? It’s an eyesore! (4)
I believe the answer is:
stye
'animal pen' is the definition.
(I've seen this before)
'at end of lane? it's an eyesore' is the wordplay.
'at' means one lot of letters go next to another.
'end of' suggests the final letters.
'it's an eyesore' becomes 'sty' (I've seen this in another clue).
The last letter of 'lane' is 'e'.
'e' after 'sty' is 'STYE'.
(Other definitions for stye that I've seen before include "Red eye" , "swelling by organ" , "Eyelid infection" , "Eye problem" , "Observer's problem" .)