there is a serial port in 8051, but i am using AT89c51, i dont think that there is any serial port in it.

Wrong, AT89C51 has a UART / serial port. on port pins P3.0, P3.1

And i am recieving synchronous data, and i have solved the problem of clock in it,

What is the nature of this synchronous data is it SPI, I2C what?

but real problem is if there is no serial port in AT89c51 then how can microcontroller recieves serial data from a standalone device

There is a serial port but as you said your data is synchronous you can't use it becuase this serial port is Asynchronous.

Regards,
Prahlad Purohit