It looks like you need to set properly the _dblclick value.
As in VFP Help:
" Note that the setting of the _DBLCLICK system variable determines how long to wait for the next letter to be typed. You may need to adjust the value of _DBLCLICK to make incremental searching work properly."

>In combo box we can search by alphabet,only with initial character.
>
>Is there any button or function to make combo box/text box for increment
>searching of table field ONE BY ONE CHARACTER.
>
>if not please suggest where can i get.