Very grown-up to avoid opening of dark cellar (5)
I believe the answer is:
vault
'dark cellar' is the definition.
'vault' can be an answer for 'cellar' (I've seen this before). I am not certain of the 'dark' bit.
'very grown-up to avoid opening' is the wordplay.
'very' becomes 'v' (common abbreviation).
'grown-up' becomes 'ult' (I am not sure about this - if you are sure you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'opening' suggests taking the first letters.
The initial letter of 'avoid' is 'a'.
'ult' put after 'a' is 'ault'.
'v'+'ault'='VAULT'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for vault that I've seen before include "Where bodies may be kept" , "Secure room -- spring over" , "Burial chamber - jump across" , "Strongroom or burial chamber" , "Secure room in bank" .)