Popular time to be cheeky at school as local muscles in (8)
I believe the answer is:
intrudes
'popular time to be cheeky at school as local muscles in' is the definition.
I can't judge whether this definition defines the answer.
'popular time to be cheeky at school' is the wordplay.
'popular' becomes 'in'.
'time' becomes 't'.
'to be cheeky at' becomes 'rude' (I can't explain this - if you can you should believe this answer much more).
'school' becomes 's'.
'in'+'t'+'rude'+'s'='INTRUDES'
Can you help me to learn more?
(Other definitions for intrudes that I've seen before include "Interferes" , "Imposes without welcome" , "Enters uninvited and unwelcome" , "Comes or joins in without being wanted" .)