Mephisto 2479

[bathtub]

I've just discovered Mephisto, and find it an enjoyable challenge on the whole - however, I'm not sure I'm quite on the wavelength yet.
Here are two for which I'm pretty sure I have the answer, but don't quite understand how it's arrived at:
18ac. Completely rejected piece about gas showing very little weight
N?NOGRA? (presumably NANOGRAM)
28ac. Puffing, perhaps, feeling old, when chasing Tom ?RO?OT?ON (presumably PROMOTION, puff in the sense of publicise)

And then one I'm less sure of:
2dn. Stop by the way, meeting Scot's grandchild
OBO? (I assume this is OBOE, but what's the Scot's grandchild got to do with it?)

Any thoughts greatly appreciated.

[novalis]

oe is an alternative spelling for oy and is a Scottish word for grandchild.
18a piece = man ( chess ) round argon ( gas ) then reversed i.e. rejected.

[novalis]

28a Tom is a slang word for a prostitute ( + pro ) Motion has the old meaning of feeling.
I don't know which dictionary you have but, if you are going to attempt Mephisto on a regular basis, I would suggest you get Chambers if you don't already have it.

[greyfox]

2dn. Stop = OBOE: OB.abbreviation for Latin 'obiter' - 'by the way' + OE Scottish for grandchild

[bathtub]

Thanks for all your help. I have Chambers - what I'm struggling with at the moment is this compiler's way of doing things. I'm sure I'll get used to the puzzle as the weeks go by and won't have to ask too many questions!