Indiscretion with girl upset sweetheart (7)
I believe the answer is:
misstep
'indiscretion' is the definition.
(thesaurus)
'with girl upset sweetheart' is the wordplay.
'with' means one lot of letters go next to another.
'girl' becomes 'miss' (term of address for an unmarried woman).
'upset' is a reversal indicator.
'sweetheart' becomes 'pet' (both are terms of endearment).
'pet' backwards is 'tep'.
'miss'+'tep' is 'MISSTEP'.
(Other definitions for misstep that I've seen before include "Error" , "Badly judged footing" , "False move" , "bad move" , "Badly judged move" .)