The affix aṭhac comes in the sense of 'employing oneself zealously in it' after the word karma in the Locative -7th case in construction.,
The word घटः means zealous work. Thus कर्मणि घटते = कर्मठः पुरुषः 'a clever, proficient person'.,
कर्मणि घटो ऽठच्
Adhyāya 5 · Pāda 2 · Rule 35