She's a bit remorseless when you get her upset (4)
I believe the answer is:
else
'she's' is the definition.
'a bit remorseless when you get her upset' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
The remaining letters 'lse' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for else that I've seen before include "In addition; different" , "Adverb meaning if not, otherwise" , "Eels (anag)" , "Other than (what is under consideration)" , "Besides; different" .)