Passionate card sent, unopened (6)
I believe the answer is:
ardent
'passionate' is the definition.
(similar in meaning)
'card sent unopened' is the wordplay.
'card' becomes 'ard' (I am not sure about this - if you are sure you should believe this answer much more).
'unopened' means to remove the first letter.
'sent' with its initial letter taken off is 'ent'.
'ard'+'ent'='ARDENT'
Can you help me to learn more?
(Other definitions for ardent that I've seen before include "Fervid" , "Zealous - ranted (anag)" , "'Fervent, impassioned (6)'" , "Warm and loving" , "Fiery and passionate" .)