Child starts to encounter maths symbol (5)
I believe the answer is:
totem
'symbol' is the definition.
(thesaurus)
'child starts to encounter maths' is the wordplay.
'child' becomes 'tot' (I've seen this before).
'starts to' suggests taking the first letters.
The first letters of 'encounter maths' is 'em'.
'tot'+'em'='TOTEM'
(Other definitions for totem that I've seen before include "Symbol given (undue) respect" , "Emblem believed to have spiritual significance" , "Venerated emblem" , "Image representing tribe" , "Pole used in ritual" .)