Vital to carry card with name to donate this? (6)
I believe the answer is:
kidney
'donate this?' is the definition.
(a kidney can be donated)
'vital to carry card with name' is the wordplay.
'vital' becomes 'key' (I've seen this in another clue).
'to carry' means one lot of letters goes inside another.
'card with' becomes 'id' (I can't justify this - if you can you should believe this answer much more).
'name' becomes 'n' (common abbreviation e.g. on forms).
'id'+'n'='idn'
'key' enclosing 'idn' is 'KIDNEY'.
'to' acts as a link.
(Other definitions for kidney that I've seen before include "body part" , "Nephritic organ" , "Sort" , "Bodily organ" , "Internal organ producing urine" .)