Darren Murphy, 26, was said to have struck lawyer John Morrison, 50, who died in Glasgow city centre just before Christmas last year.
The claim was made as Mr Murphy went on trial at the High Court in Glasgow charged with the culpable homicide of Mr Morrison.
He denies the accusation and has lodged special defences of self defence and incrimination.
A jury was told how Mr Murphy and his friends had been on their way to the a nightclub at the time of the alleged killing in the early hours of December 22.
The trial heard Mr Morrison, ex-Lib Dem leader of East Dunbartonshire Council, had earlier been on a works night out.
One of Mr Murphy's friends later allegedly pushed Mr Morrison, who was then sworn at.
Witness Samantha Mulgrew, 21, said she had been walking with Murphy in Glasgow's Argyle Street when Mr Morrison - whom she did not know - gave her a fright by coming up close to her.
Prosecutor Paul Kearney asked Miss Mulgrew what Mr Murphy did.
She broke down before telling the jury that the accused "hit the man".
Mr Morrison died in the city's Dunlop Street close to the St Enoch Centre.
The court was told Mr Murphy was "really upset" when he learned of the death.
The trial, before Judge Norman Ritchie, QC, continues.