Letter from Greece united Kelvin with father in Chennai (5)
I believe the answer is:
kappa
'letter from greece' is the definition.
(letter from the Greek alphabet)
'united kelvin with father in chennai' is the wordplay.
I cannot quite see how this works, but
'kelvin' could be 'k' (abbreviation for the unit of temperature) and 'k' is found within the answer.
'father' could be 'pa' (pa can mean dad or father) and 'pa' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kappa that I've seen before include "Letter before lambda" , "Tenth Greek letter" , "Greek K" , "Greek character" , "letter from abroad" .)