Tell Rex to cheer up (6)
I believe the answer is:
relate
'tell' is the definition.
(as in to tell or relate a story)
'rex to cheer up' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'to cheer up' becomes 'elate' (I've seen this in another clue).
'r'+'elate'='RELATE'
(Other definitions for relate that I've seen before include "Do well; wave about" , "Tell; connect" , "Marriage Guidance - report" , "Tell; a counselling service" , "Connect in Tralee" .)