She gains an advantage if upset (5)
I believe the answer is:
tessa
'she' is the definition.
'an advantage if upset' is the wordplay.
'an advantage' becomes 'asset' (I've seen this before).
'if upset' says the letters should be written in reverse (down clues only).
'asset' in reverse letter order is 'TESSA'.
'gains' acts as a link.
(Other definitions for tessa that I've seen before include "Girl" , "Tax-exempt special savings account" , "Dahl or Jowell" , "Girl's name; former savings account" , "Woman" .)