It is very unlikely that the clutch is causing poor fuel economy.
But the Galaxy is not a small vehicle, if you are doing lots of short runs in rush-hour traffic, then 20mpg is all you might reasonably get.
If on the other hand you are only getting 20mph when, for the majority of the time, the vehicle is in fifth gear – then I would have a garage take a look into the issue (check the exhaust gases, plugs, injectors, possible binding brakes etc). If something is wrong, causing excessive fuel consumption, any garage should be able to spot it and take corrective action (for a fee).