Horrid adult breaking new pen (5)
I believe the answer is:
nasty
'horrid' is the definition.
(I've seen this before)
'adult breaking new pen' is the wordplay.
'adult' becomes 'a' (abbreviation).
'breaking' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'pen' becomes 'sty' (both can mean an animal enclosure).
'n'+'sty'='nsty'
'a' placed into 'nsty' is 'NASTY'.
(Other definitions for nasty that I've seen before include "Mean" , "rank" , "Beastly" , "Awful" , "Repulsive" .)