Thanks, Roy. There are no other baptisms in the parish with John Clark(e) as a father, and no marriages in Sussex between a John Clark and Mary (the parents of John Clarke are John Clarke and Mary according to the transcript). There is, however, a marriage between Thomas Clarke and Mary, and four baptisms of children of Thomas Clarke and Mary between 1763, 1764, 1766, and 1772, leaving (as you can see) a nice gap for John in 1769. All of this is what made me ask in the first place.
Interestingly, there's also a William Clark that's apparently a son of William Clark and Mary in 1767, which also doesn't fit (again there's no marriage), so I wonder whether either the transcriber or the priest (or even the parents) made the same error on both occasions, putting the son's name into the father's column. It would be nice to know what the original said, though.