'pro-clergy' is a modern compound formed from the prefix 'pro-' (from Latin 'pro' meaning 'for' or 'in favor of') plus 'clergy' (from Old French 'clergie' and Latin 'clericus' meaning 'clergyman').

'clergy' changed from Old French 'clergie' (from Latin 'clericus', ultimately from Greek 'klērikos') and became the modern English 'clergy'; the compound 'pro-clergy' is a recent English formation combining 'pro-' with that noun.

Initially, 'clergy' meant 'those set apart for religious service' and later came to mean 'ordained religious leaders'; the prefix 'pro-' has retained the meaning 'for', so 'pro-clergy' now means 'for or supportive of the clergy'.