Hi Northbeach,
"maintain 280kias when able" =
I would interpret that as increase to 280 if below 10 and to not exceed 280 if above 10.
Whatever clearance ATC gives you, if there is no condition attached, then you are expected to oblige right away.
The actual phraseology for these instructions will vary alot from unit to unit as standard phraseology is still far from reality in the US..
Icao Annex 10 is worth a read