The girl, alas, in retreat, letting out the odd cry (5)
I believe the answer is:
aargh
'cry' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
(Other definitions for aargh that I've seen before include "Cry of exasperation!" , "Yell of dismay" , "that's dreadful!" , "Exclamation of frustration or rage" , "Cry of utter frustration" .)