Flame once right to stop boiling egg on (6)
I believe the answer is:
exhort
'egg on' is the definition.
(I've seen this in another clue)
'flame once right to stop boiling' is the wordplay.
'flame once' becomes 'ex' (an ex is an old flame).
'right' becomes 'r' (common abbreviation).
'to stop' is an insertion indicator.
'boiling' becomes 'hot' (similar in meaning).
'r' put inside 'hot' is 'hort'.
'ex'+'hort'='EXHORT'
(Other definitions for exhort that I've seen before include "Entreat earnestly, urge" , "Spur on" , "Urge, advise earnestly" , "Strongly urge (someone to do something)" , "Urge strongly and sincerely" .)