Denoting a numerical order in a series (7)
I believe the answer is:
ordinal
'denoting a numerical order in a series' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ordinal that I've seen before include "Numerals such as first and second" , "Type of number" , "Number which shows position in a series" , "RC Church service book" , "It shows your place" .)