Fumblestep says that at any point during the spells duration you can as a standard action discharge the spell and perform a trip maneuver.
My question is can you cast and trigger the effect in one action? That would make it a much more useful spell.
No, you definitely can't. Casting the spell is a standard action. Discharging the spell is a standard action. There is nothing in the game that allows you to take two standard actions in one round or to combine two standard actions into one (or if there is, it's incredibly limited).
Or looking at it another way, the duration of the spell (during which you can perform the trip) doesn't start until right after the casting time is finished. And at that point, you have already spent your standard action to cast the spell.
Hmm, that's a good argument thanks.
I found another spell: thunderstomp does what I want so I won't need Fumblestep.