Rulers, slide rules, algebra, statistics, trigonometry — all initially rejected (5)
I believe the answer is:
tsars
'rulers' is the definition.
(tsar is a kind of ruler)
'slide rules algebra statistics trigonometry all initially rejected' is the wordplay.
'all initially' suggests taking the first letters (all the initial letters).
'rejected' says the letters should be written in reverse.
The first letters of 'slide rules algebra statistics trigonometry' is 'srast'.
'srast' back-to-front is 'TSARS'.
(Other definitions for tsars that I've seen before include "Old Russian rulers" , "Old Russia's rulers" , "Old monarchs" , "Russian emperors" .)