Prediction favouring actors on Eastenders initially (8)
I believe the answer is:
Here is my best explanation:
'prediction' is the definition.
(forecast is a kind of prediction)
'favouring actors on eastenders initially' is the subsidiary indication.
'favouring' becomes 'for' (I've seen this in other clues).
'actors' becomes 'cast'.
'on' is a charade indicator (letters next to each other).
'initially' says to take the initial letters.
The first letter of 'eastenders' is 'e'.
'cast' put after 'e' is 'ecast'.
(Other definitions for forecast that I've seen before include "tip", "Fear Scot in prediction", "Divine", "Prediction, about the weather perhaps", "Predict (event)", "Predict as with weather", "Weather prediction", "Meteorological surmise perhaps", "Prophecy" and "Tell in advance what will happen".)