Learn to tell rumour (7)
I believe the answer is:
hearsay
'rumour' is the definition.
(I know that rumour can be written as hearsay)
'learn to tell' is the wordplay.
'learn' becomes 'hear' (synonyms).
'to tell' becomes 'say' (synonyms).
'hear'+'say'='HEARSAY'
(Other definitions for hearsay that I've seen before include "Common talk" , "Kind of rumour, makes bad evidence" , "rumoured" , "Gossip or idle talk - it's not evidence" , "product of grapevine" .)