Inmates don't have to join the competition (5)
I believe the answer is:
enter
'join the competition' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for enter that I've seen before include "Enlist, join up" , "Fill in" , "Set foot in" , "Enrol" , "Make a record of" .)