Discriminating person is engaged in a purchase (6)
I believe the answer is:
ageist
'discriminating person' is the definition.
(I've seen this in another clue)
'is engaged in a purchase' is the wordplay.
'engaged in' means one lot of letters goes inside another.
'purchase' becomes 'get'.
'a'+'get'='aget'
'is' inserted into 'aget' is 'AGEIST'.
(Other definitions for ageist that I've seen before include "Discriminating on the basis of years lived" , "being for youngsters only?" , "One prejudiced on grounds of age" , "One discriminating against those who may be getting on?" , "Discriminating against older persons" .)