Clue: His forecast originally was wrong (4)
Answer: FISH
Explanation: Definition: His forecast originally was wrong
Anagram of HIS + F[orecast] – I imagine this will have come as a total mystery to many, but it refers to the UK's Great Storm of 1987, and a weather forecast the previous day in which Michael Fish told people not to worry about claims that a hurricane was on the way, though he later claimed he was misquoted or misunderstood. As well as WEATHER FORECAST at 18a, there are numerous references throughout puzzle to both weather conditions and forecasts