Finally clean a pig enclosure that's unpleasant! (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
('nasty' can be a synonym of 'unpleasant')
'finally clean a pig enclosure' is the wordplay.
'finally' indicates one should take the final letters.
'pig enclosure' becomes 'sty'.
The last letter of 'clean' is 'n'.
'n'+'a'+'sty'='NASTY'
'that's' is the link.
(Other definitions for nasty that I've seen before include "Highly unpleasant" , "Mean" , "Disagreeable" , "Offensive or malicious" , "One who sees the future" .)