I can't duplicate your results. The formula
=TIMEVALUE(A1)
produces the Excel serial number for the time which can then be formatted as time.
Your question indicates that there is nothing else in the cell than the time as text. It would be helpful to actually see a screenshot of the data to confirm that there isn't something interfering with the formula. Also, make sure that the format is set to General or Time before entering the formula.
As an alternative you can try this formula, although if there is something keeping the TIMEVALUE formula from working it may keep this from working as well.
=TIME(TEXTBEFORE(A1,":"),TEXTAFTER(A1,":"),0)