Severely paring its investment portfolio of junk bonds and poor real estate investments, USF&G; Corp., the giant Baltimore insurance company, today reported a loss of $610 million in the fourth quarter and further chopped its quarterly common stock dividend, this time to a nickel a share.

The news sparked an imbalance of sellers and buyers of USF&G; stock and the New York Stock Exchange delayed trading of the stock this morning.

The insurance giant said the $610 million loss amounted to $7.32 a share compared with a profit of $104 million or $1.20 a share in the year ago period. More than half of the red ink -- $357 million, or $4.25 a share -- came from losses from selling and restructuring investments.

For all of 1990, USF&G; reported a loss of $569 million, or $6.99 a share, compared with a profit of $119 million, or $1.24 per share, in 1989.

The company cut its dividend to 5 cents per share, payable on April 30.

Along with clearing the company's investment portfolio of problem investments, Norman P. Blake, Jr., chairman and chief executive officer of USF&G;, said the company will get out of the leasing, travel services and other "non-core" businesses.