Very serious item in the post about it’s recalled (7)
I believe the answer is:
drastic
'very serious' is the definition.
(drastic means very severe or serious)
'item in the post about it's recalled' is the wordplay.
'item in the post' becomes 'card' (eg a birthday card).
'about' means one lot of letters goes inside another.
'recalled' says the letters should be written in reverse.
'card' placed around 'its' is 'citsard'.
'citsard' back-to-front is 'DRASTIC'.
(Other definitions for drastic that I've seen before include "Harsh, severe" , "Extremely severe" , "Unsparing" , "Grave" , "Uncompromising" .)