Girls one notices being absent (6)
I believe the answer is:
misses
'girls' is the definition.
('miss' can be a synonym of 'girl')
'one notices being absent' is the wordplay.
'one' becomes 'mis' (I can't explain this - if you can you should give a lot more credence to this answer).
'notices' becomes 'senses'.
'being' becomes 'ens' (philosophical term for being or existence).
'absent' is a deletion indicator.
'senses' with 'ens' taken away is 'ses'.
'mis'+'ses'='MISSES'
Can you help me to learn more?
(Other definitions for misses that I've seen before include "Fails to meet" , "young women" , "Failures to hit" , "Young ladies; avoids" , "Unsuccessful attempts" .)