Man's penetrating stare unsettled girl (7)
I believe the answer is:
theresa
'girl' is the definition.
'man's penetrating stare unsettled' is the wordplay.
'man' becomes 'he'.
'penetrating' means one lot of letters goes inside another.
'unsettled' indicates anagramming the letters.
'stare' anagrammed gives 'tresa'.
'he' put within 'tresa' is 'THERESA'.
(Other definitions for theresa that I've seen before include "May's one" , "Maria -, eighteenth century empress" , "Maria -, Habsburg empress" , "Woman" , "-- May, MP" .)