Guard, not small, right to go in (5)
I believe the answer is:
entry
'right to go in' is the definition.
I can't judge whether this definition defines the answer.
'guard not small' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'not' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sentry' with 's' taken out is 'ENTRY'.
Can you help me to learn more?
(Other definitions for entry that I've seen before include "Competitors - way in" , "Gate" , "Competitor" , "all runners" , "candidate" .)