Initially rated zero, below one who runs all bases (7)
I believe the answer is:
rounder
'initially rated zero below one who runs all bases' is the definition.
I can't tell whether this definition defines the answer.
'initially rated zero below' is the wordplay.
'initially' says to take the initial letters.
'zero' becomes 'O' (O resembles 0).
'below' becomes 'under' (synonyms).
The initial letter of 'rated' is 'r'.
'r'+'o'+'under'='ROUNDER'
Can you help me to learn more?
(Other definitions for rounder that I've seen before include "Score in bat and ball game" , "Score at baseball-type game" , "More circular" , "Or under (anag.)" , "More spherical" .)