As part of the changes, Citigate's independent worldwide offices will be brought together to form a global coordinated brand under a single management team.
Longstanding chief executive Clare is to become deputy chair of a new holding company named CDR Holdings and will step down from the board of the brand he helped establish in 1988.
He will continue with his client responsibilities out of the London office.
It is also understood chief operating officer Paul Baker has been made redundant as part of the reorganisation.
Meanwhile, Huntsworth chief executive Lord Chadlington will step in to take a more hands-on role at the agency.
Chadlington will take on the role of non-executive chairman of CDR Holdings, while Huntsworth group business development director Alison Clarke will also join the board.
A search is under way to find a new group CEO for the agency, but industry sources said there were few obvious names internally to step into Clare's shoes. It is thought an external appointment will be confirmed in the coming months.
The firm said it remained committed to expanding the network by acquisitions, start-ups or funding new teams and bolstering underweight offices in the network with new hires.
CDR will continue its longstanding policy of dividing its business into three separate divisions: financial, corporate and public affairs.
It is understood that Huntsworth's other financial brand, Hudson Sandler, will continue as an independent entity.
Citigate has fee revenues of £27m globally and employs more than 200 people in offices across London, continental Europe, Asia-Pacific and an associated company in North America.