Mark of extended union membership apparently on the cards? (7,7)
I believe the answer is:
diamond wedding
'mark of extended union membership' is the definition.
(occasion marking 60 years of marriage)
'apparently on the cards?' is the wordplay.
I cannot really understand how this works, but
'on' is present in the answer.
This explanation may well be incorrect...
(Other definitions for diamond wedding that I've seen before include "family celebration" , "big union anniversary?" , "celebration of the elderly?" , "Sixtieth anniversary of marriage" , "celebration for pensioners" .)