Girl upset with a troubled soul - how irregular (9)
I believe the answer is:
anomalous
'irregular' is the definition.
(I've seen this before)
'girl upset with a troubled soul' is the wordplay.
'girl' becomes 'mona' ().
'upset' says the letters should be written backwards.
'with' indicates putting letters inside.
'troubled' is an anagram indicator.
'mona' back-to-front is 'anom'.
'soul' anagrammed gives 'lous'.
'a' going into 'lous' is 'alous'.
'anom'+'alous'='ANOMALOUS'
'how' acts as a link.
(Other definitions for anomalous that I've seen before include "Atypical, exceptional" , "Inconsistent, deviating from the norm" , "Abnormal, irregular" , "Deviating from normal" , "Aberrant" .)