Union woman at first liked using arithmetic values (9)
I believe the answer is:
numerical
'using arithmetic values' is the definition.
(I've seen this before)
'union woman at first liked' is the wordplay.
'union' becomes 'NUM' (National Union of Mineworkers).
'woman' becomes 'erica' ().
'at first' indicates taking the first letters.
The first letter of 'liked' is 'l'.
'num'+'erica'+'l'='NUMERICAL'
(Other definitions for numerical that I've seen before include "Relating to quantity" , "Using arithmetic values" , "like characters, perhaps" , "expressed in figures" , "Relating to such as 9 or 123" .)