Terriers, on average, are domesticated (5)
I believe the answer is:
tamed
'domesticated' is the definition.
(similar in meaning)
'terriers on average are' is the wordplay.
'terriers' becomes 'ta' (nickname for Territorial Army members).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'average are' becomes 'med' (I can't explain this - if you can you should believe this answer much more).
'ta'+'med'='TAMED'
Can you help me to learn more?
(Other definitions for tamed that I've seen before include "Of animals, domesticated" , "Reined" , "Domesticated or subdued" , "now behaving himself?" .)