Idea is, he gets the ball in: a score (6)
I believe the answer is:
theory
'idea' is the definition.
(I've seen this before)
'he gets the ball in a score' is the wordplay.
'gets' is a charade indicator (letters next to each other).
'the ball' becomes 'o'.
'in' indicates putting letters inside.
'a score' becomes 'try' (**).
'he'+'o'='heo'
'heo' going into 'try' is 'THEORY'.
'is' acts as a link.
(Other definitions for theory that I've seen before include "can provide an explanation" , "Speculation" , "Opinion or supposition to explain something" , "Idea to explain something, say evolution" , "Suggested explanation" .)