Symbol seen in general mathematics appears initially (5)
I believe the answer is:
sigma
'symbol' is the definition.
(sigma is a kind of symbol)
'seen in general mathematics appears initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'seen in general mathematics appears' is 'SIGMA'.
(Other definitions for sigma that I've seen before include "Gr. letter; rhymes with mark of disgrace" , "Letter from Greece" , "Greek letter used as summation sign" , "Foreign character" , "character in Corfu" .)