The tail of a sheep, I see, is very long (4)
I believe the answer is:
epic
'very long' is the definition.
(I've seen this before)
'the tail of a sheep i see' is the wordplay.
'the tail of a sheep i' becomes 'epi' (I can't justify this - if you can you should believe this answer much more).
'see' becomes 'C' (the word for the letter, according to Chambers).
'epi'+'c'='EPIC'
'is' is the link.
Can you help me to learn more?
(Other definitions for epic that I've seen before include "great film" , "Ben Hur, say" , "Long poem - awesome!" , "Impressively great" , "Colossal" .)