Dogs are best trained when small in it (7)
I believe the answer is:
bassets
'dogs' is the definition.
(basset is a kind of dog)
'best trained when small in it' is the wordplay.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'when' becomes 'as'.
'small' becomes 's' (abbreviation - e.g. clothes size).
'in it' is an insertion indicator.
'as'+'s'='ass'
'best' with letters rearranged gives 'bets'.
'bets' placed around 'ass' is 'BASSETS'.
'are' acts as a link.
(Other definitions for bassets that I've seen before include "Dogs" , "Hounds with short legs and long ears" .)