Sincere start - return Emmy and attempt balance (8)
I believe the answer is:
symmetry
'balance' is the definition.
(synonyms)
'sincere start return emmy and attempt' is the wordplay.
'start' suggests taking the first letters.
'return' shows that the letters should be reversed in order.
'and' means one lot of letters go next to another.
'attempt' becomes 'try' (synonyms).
The first letter of 'sincere' is 's'.
'emmy' back-to-front is 'ymme'.
's'+'ymme'+'try'='SYMMETRY'
(Other definitions for symmetry that I've seen before include "Property of this grid" , "Balance between two parts" , "shapely attribute" , "Regularity of shape" , "Pleasing proportion between parts of a whole" .)