Yeah ... thanks everyone. Mis-types apart(!) I can now see that it's in the clue not once, but twice twice. Initial letters of "is reached as marauders invade" give IRAMI; and so does "prIMARIly" if read "from the east" (i.e. from the rh side of the word). Clever stuff .... but, for all that, none of my dictionaries contains the word!