Trained terrier, one who is out of service (7)
I believe the answer is:
retirer
'one who is out of service' is the definition.
The definition suggests a singular noun which matches the answer.
'trained terrier' is the wordplay.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'terrier' is an anagram of 'RETIRER'.
Can you help me to learn more?
(Other definitions for retirer that I've seen before include "One going to bed" , "he goes to bed" .)