The favourite, when in form (8,3)
I believe the answer is:
teachers pet
'the' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'favourite when in form' is the wordplay.
I cannot quite see how this works, but
'favourite' could be 'pet' (synonyms) and 'pet' is found in the answer.
The remaining letters 'teachers' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for teachers pet that I've seen before include "Class favourite" , "Favoured pupil" , "(Mocked) school favourite" , "goody-goody" , "Favoured child at school" .)