The favourite, when in form (8,3)
I believe the answer is:
teachers pet
'form' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'the favourite when' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'favourite' could be 'pet' (synonyms) and 'pet' is found within the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
This explanation may well be incorrect...
'in' acts as a link.
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" .)