The affix ap comes after the verb ad 'to eat' when an upasarga is in composition.,
As विघसः (11. 4. 38) 'half-chewed morsel'; प्रघसः 'voracious'.
Why do we say 'when compounded with an upasarga'? Observe घासः 'grass'; अद् + घञ् = घस् + अ 2.4.38.,
उपसर्गे ऽदः
Adhyāya 3 · Pāda 3 · Rule 59