I'm puzzled by the phrase 'one ambiguity'. The unclued entry it is meant to indicate isn't ambiguous - what else could it be? In defining it as 'one', the setter seems to be thining that we'll also be flummoxed by e.g. NCARAGUA. Surely he should have said 'one deficiency'?
Dysart is usually a very careful setter but he seems to have made several blunders in this preamble.