Between posts, perhaps, he serves as custodian (6)
I believe the answer is:
keeper
'custodian' is the definition.
(I know that custodian can be written as keeper)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for keeper that I've seen before include "One guarding goal" , "Goalie" , "slips on my right hand usually" , "One at the wicket" , "Reluctant" .)