Complex sentence: After demitting office, he had initially expressed reservations against the Indo-US civilian nuclear deal following which the Prime Minister Dr. Manmohan Singh had him briefed specially to address the concerns Mr. Mishra had about the deal.
Split into simpler sentences: After demitting office, Mishra had initially expressed reservations against the Indo-US civilian nuclear deal. Following this, then Prime Minister Dr. Manmohan Singh briefed specially to address his concerns about the deal.