Start to care about opening for group of sisters (7)

I believe the answer is:
convent
'group of sisters' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'start to care about opening for' is the wordplay.
'start to' says to take the initial letters.
'about' becomes 'on' (as in a book 'on' a certain topic).
'opening for' becomes 'vent' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'care' is 'c'.
'c'+'on'+'vent'='CONVENT'
Can you help me to learn more?
(Other definitions for convent that I've seen before include "Religious community" , "Community of nuns" , "place for nuns" , "Building for nuns" , "Religious house" .)
