Finally clean a pig pen that's horrid (5)
I believe the answer is:
nasty
'horrid' is the definition.
(I've seen this before)
'finally clean a pig pen' is the wordplay.
'finally' says to take the final letters.
'pig pen' becomes 'sty' (where pigs are kept).
The last letter of 'clean' is 'n'.
'n'+'a'+'sty'='NASTY'
'that's' acts as a link.
(Other definitions for nasty that I've seen before include "Disagreeable" , "Horrid, repugnant" , "Filthy" , "rank" , "Vile" .)