Telegraph ends, right, with setter's flourish (6)
I believe the answer is:
thrive
'flourish' is the definition.
('thrive' can be a synonym of 'flourish')
'telegraph ends right with setter's' is the wordplay.
'ends' means to remove the middle letters.
'right' becomes 'r' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'setter's' becomes 'ive' (I've seen this in other clues**).
'telegraph' with its middle taken out is 'th'.
'th'+'r'+'ive'='THRIVE'
(Other definitions for thrive that I've seen before include "Prosper, grow well" , "Grow strongly" , "Prosper, flourish" , "Boom" , "Develop well" .)