Insulting remark, so low, upset rector initially (4)
I believe the answer is:
slur
'insulting remark' is the definition.
(I've seen this in another clue)
'so low upset rector initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'so low upset rector' is 'SLUR'.
(Other definitions for slur that I've seen before include "Pronounce indistinctly or speak ill of" , "Slight, insult" , "Defamation" , "Stigma" , "Disparaging remark" .)