It's hard to hide a key, one learned (5)
I believe the answer is:
heard
'one learned' is the definition.
(I've seen this in another clue)
'it's hard to hide' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'to hide' indicates putting letters inside.
'e' put within 'hard' is 'HEARD'.
'a key' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for heard that I've seen before include "Perceived sound" , "Received aurally" , "Was told of" , "Attended to" , "Gathered" .)