When route-refresh capability is negotiated between two BGP peers , "clear ip bgp *" will not trigger a hard-reset at all. Is it right to say that hard reset never happens with router with route-refresh capabilities ?
No, that command will trigger a hard-reset of all peerings.
Negotiating route-refresh means you can perform a soft-reset using, for example
Note that soft-reconfiguration inbound is a waste of RAM and should generally be avoided if route-refresh capability is available - the only exceptions are where the time/bandwidth consumption is considered more important.
If you want to be able to trigger a hard reset without loss of forwarding, look at BGP graceful-restart