A cricket ground was the entirety of the playing area in cricket. The Fifth Doctor implied, however, that the ground was principally the area where fielding occurred, because he directed Peri and Erimem to look for Tegan "in the ground somewhere", and then rattled off a series of exclusively fielding locations where the Australian might be found. (PROSE: Graham Dilley Saves the World)
Behind the scenes
Though never made crystal clear in Doctor Who fiction, the ground is the whole, circular playing area, including the smaller inner circle of the cricket pitch, where bowling and batting occur. However the Fifth Doctor's instruction to look for Tegan "in the ground somewhere" means to ignore the pitch and look in the larger fielding area.