Put the screws on firm, marble-top gym locker for beginner (6)
I believe the answer is:
compel
'locker for beginner' is the definition.
I can't judge whether this definition defines the answer.
'put the screws on firm marble-top gym' is the wordplay.
'put the screws' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'firm' becomes 'co' (a firm is a company).
'marble-top' becomes 'm' (top of the word 'marble-').
'gym' becomes 'pe' (short for Physical Education).
'co'+'m'+'pe'='compe'
'l' put after 'compe' is 'COMPEL'.
Can you help me to learn more?
(Other definitions for compel that I've seen before include "Coerce, force to obey" , "Bring about by force" , "Use force or influence to make person do something" , "'Force, oblige (6)'" , "Use force, oblige" .)