Custodian's ringful (4)
I believe the answer is:
keys
'custodian's ringful' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for keys that I've seen before include "Lock-openers?" , "Islands; piano parts" , "Low islands, sycamore fruits" , "Solutions" , "Bunch of openers?" .)