I was thinking of using WD40 to clean my amp's circuit board.will it be ok? wont it damage the transistors or the board itself?
WD40 leaves a substance behind that is technically known as "evil fookin sludge" that attracts and holds dirt. Do not use it for anything in electronics unless you hate it and are just looking for something to start a fire on it with.

What are you trying to clean off the circuit board?
If it was the usual sort of spill like a beverage, you'll need to start with a water based cleaner, like 409 and a small paint brush or old tooth brush. Rinse with clean water, blow or sling dry most of the water off and finish with a blow drier or over a heater vent for a day or so.

For flux, or other mung from working on it use alcohol or acetone and a small "flux" or "acid" brush.

Anything else?