Q.Explosimeter measures gas concentration as percentage of LEL by actual combustion of the gas.How is the combustion possible if concentration is below the LEL?

You are confusing the term "Combustion" (Burning) with "Explosion". LEL is used to define if the gas concentration will explode if a source of ignition is provided.

If the Gas concentration is below the 100% of LEL range (approx 1% of Volume for methane), it will not explode even when a source of igntion is provided.

But as the gas mixture will still have small amount of flammable gas in it, it can burn if it is continuously passed through a heated surface. The extent of burning (or combustion) will depend upon the amount of flammable gas in the gas mixture.

Explosimeter uses the principle of wheatstone bridge in which one resistor is in form of a heated element. The more it gets heated more resistance it will provide in the circuit. This will generated a current flow in the circuit which can be measured.

In case of Explosimeter, the gas sample is passed through this heated element. More flammable gas (in term of % LEL) the sample has, more it will burn (not explode) and more heated the element will become. 

As the resistance of other three resistors in the circuit is known and is fixed, the resistance for the fourth resistor (which varies as per the flammable gas concerntartion) and the current flow generated because of this imbalance of resistance in the circuit is calculated.

The %LEL in the gas sample required for the generated current flow is shown in form of a reading on the explosimeter.