Helping the pooris a great virtue. Here the underlined part is a/an -

• Helping the poor is a great virtue. Here the underlined part is a - Gerundial Phrase.
• Gerundial Phrase: Verb+ing + Extension যদি Noun (Subject/ Object) এর কাজ করে তবে তাকে Gerundial Phrase বলে।
যেমন:
- Walking in the morning is good for health.
- Helping the poor is a great virtue.
- Stop telling lies. 
- Cycling on the hill tracks is not easy.

• এখানে, 'Helping the poor' phrase টি বাক্যের subject বা noun এর কাজ করছে তাই এটি Gerundial Phrase.