fare qualcosa "under the wire" significa farlo senza essere scoperto, farlo in un modo occulto "slip" in questa orazione penso che significa "scivolare" nel senso di "far passare" quindi "i slipped it right under the wire" significa "lo ho passato/scivolato senza essere scoperto"

"To do something under the wire" means "to do something at the last possible moment". The phrase "slipped it in" can have various meanings. In this case, it probably refers to submitting a piece of work / essay very close to the deadline. I don't know where the phrase "under the wire came" from. Maybe someone else can explain? As for the phrase "slipped it in": People are sometimes described as "slipping in" through doorways and windows when there is only a small gap, or the door/window is in the process of being closed. In the case of a submitting a piece of work, the meaning is very similar: there is only a short amount of time remaining, the "window of opportunity" ( <- a very common metaphor) is very small.