He's telling all, having had a love upset (8)
I believe the answer is:
franklin
'he's telling all' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'having had a love upset' is the wordplay.
'having' becomes 'frank' (I can't explain this - if you can you should give a lot more credence to this answer).
'had' says to put letters next to each other.
'a love' becomes 'nil' (love and nil mean zero in cricket and football).
'upset' says the letters should be written in reverse.
'nil' in reverse letter order is 'lin'.
'frank'+'lin'='FRANKLIN'
Can you help me to learn more?
(Other definitions for franklin that I've seen before include "Tale-bearer" , "[PILGRIM in the CANTERBURY TALES]" , "Benjamin or Roosevelt" , "old landowner" , "Statesman" .)