Rare shock: Charlie allowed in (6)
I believe the answer is:
scarce
'rare' is the definition.
(similar in meaning)
'charlie allowed in' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'allowed' becomes 'scare' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'c' put inside 'scare' is 'SCARCE'.
'shock' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for scarce that I've seen before include "Not enough to meet demand" , "Falling short of demand" , "Insufficient to meet demand" , "Few and far between" , "Not plentiful, hard to find" .)