Wishes they were here - the girls, that is (6)
I believe the answer is:
misses
'girls that is' is the definition.
'misses' can be an answer for 'girls' ('miss' can be a synonym of 'girl'). I am not certain of the rest of the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for misses that I've seen before include "Overlooks" , "Failures to hit" , "Doesn't hit; young ladies" , "young women" , "Unmarried ladies" .)