The definition of onside is in a position to play.(adjective)
An example of onside used as an adjective is the phrase an "onside position" which means a player is in a spot to get the ball.
Onside is defined as a sports term that means in the proper position to play.(adverb)
An example of onside used as an adverb is the phrase an "onside move" which means a move that allows the player to get the ball.
See onside in Webster's New World College Dictionary
See onside in American Heritage Dictionary 4
adverb & adj.