Generally speaking in allotment terms 'clearing' means pretty much what it says on the tin, getting yourself some clear space to enable further cultivation to proceed unhindered. This can mean different things in different contexts but for a neglected plot would normally mean gaining access to the soil surface, locating boundaries etc. On an established plot could simply mean removing the remains of crops at the end of the season.
Regarding a glossary of terms, I think this is a good idea and as I have a number of websites with database facilities, I may consider setting a page up next winter. Meantime perhaps someone would like to assemble a list of lottie terms.