Urge in old flame burning: that's about right (6)
I believe the answer is:
exhort
'urge' is the definition.
(I know that urge can be written as exhort)
'old flame burning that's about right' is the wordplay.
'old flame' becomes 'ex' (a former partner).
'burning' becomes 'hot' (similar in meaning).
'that's about' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'hot' enclosing 'r' is 'hort'.
'ex'+'hort'='EXHORT'
'in' is the link.
(Other definitions for exhort that I've seen before include "Strongly urge (someone to do something)" , "Urge, counsel earnestly" , "Urge, advise earnestly" , "Call" , "Strongly encourage (someone to action)" .)