Only few will be admitted when you find the key (5)
I believe the answer is:
solve
'you find the key' is the definition.
I can't tell whether this definition defines the answer.
'only few will be admitted' is the wordplay.
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'few will' becomes 'v' (I can't explain this - if you can you should give a lot more credence to this answer).
'be admitted' means one lot of letters goes inside another.
'sole' enclosing 'v' is 'SOLVE'.
'when' is the link.
Can you help me to learn more?
(Other definitions for solve that I've seen before include "Complete crossword" , "Find the answer to this clue, for instance" , "do crosswords" , "Settle" , "Find the solution to puzzle or problem" .)