One on the pitch who walks out (7)
I believe the answer is:
striker
'out' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one on the pitch who walks' is the wordplay.
'one on' says to put letters next to each other.
'the pitch' becomes 'strike' (strike is a kind of pitch).
'who walks' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'strike' put next to 'r' is 'STRIKER'.
Can you help me to learn more?
(Other definitions for striker that I've seen before include "disaffected employee" , "he's still batting" , "Football player" , "Football attacker" , "one who's out?" .)