I am sort of new to these forums and Gaelic learning and have a question that has been burning for a while.
I know that the word 'am flat' is masculine and that in the genitive lenition should occur so would that mean it would become; 'an fhlat'?
Such as in the phrase; Buidheann an fhlat - the group the of the flat?
Yep. Masculine nouns exhibit the same lenition in the genitive as in the dative. There is also a general rule that they slenderise a final broad vowel sound as in an cat -> a' chait, but that is not indicated in Am Faclair Beag, so I'd say that an fhlat is correct.
It's in a way a bad example because it's a recent loanword and doesn't necessarily follow the same/right rules. But even historically, some words don't show inflection for the genitive. In terms of lenition, with f in loanwords that seems to be a bit of a thorny issue so avoiding flat as a loanword and using furan, buidheann + furan would indeed yield buidheann an fhurain.
'S e cuspair inseanach a th'ann. I'm happy to accept that "flat" is a borrowed word and this raises the question of
whether or to what extent such words should follow perceived conventions. I would say also there may be a question as to whether the consonant combination "fl" at the start of a Gàidhlig word would trigger lenition. I'm off to do some
research into initial clusters and I'll let you know how I get on.