If the other gears select ok then I wouldn't think it was the clutch. First things to check before you go into serious problem mode is check the linkage and bushings. I currently am replacing my gearbox after finding first gear synchro teeth in the oil. If the gate isn't holding the car in gear it could be wear or misalignment of a linkage component , there are a few. Try these things first.
Check the plastic bushing at the base of the gear stick, they are prone to cracking and wear, while there check the general appearance of the stop plate in particular and the other parts in there, you will need a manual. I have a diagram if you want me to email it. Open the tunnel plate in the floor behind the driver's seats that gives access to the coupler bushings to the gearbox input shaft. Make sure that everything is like new in there and tightened up properly, if you start taking things apart remember to mark the position of the gear stick shaft and the gearbox input shaft.
If all that checks out drain your gearbox and examine the contents for metal particles, There shouldn't be any! If that's ok refill with Swepco201 you can get it here.
http://www.berlyn-services.co.uk/parts.htm
With the car stationery and the engine off go through the gears especially first and check the 'feel' with the stick of your work. If it still pops out then it's looking a little more like the box but try those things and post again.
