Sorry a girl's lost, that is (3)
I believe the answer is:
sad
'sorry a girl's lost that is' is the definition.
'sad' can be an answer for 'sorry' (I've seen this in another clue). I'm not sure about the rest of the definition.
'sorry a girl's lost' is the wordplay.
'sorry' indicates an anagram (sorry can mean poor-quality or wretched).
'girl's' becomes 'ada's' (girl's name).
'lost' suggests deleting specific letters.
'adas' with 'a' taken out is 'das'.
'das' anagrammed gives 'SAD'.
Can you help me to learn more?
(Other definitions for sad that I've seen before include "grey" , "Despondent" , "Dejected" , "Deplorable" , "Doleful" .)