Cardinal, say, is back holding a letter-opener (4)
I believe the answer is:
dear
'letter-opener' is the definition.
I can't tell whether this defines the answer.
'cardinal say is back holding a' is the wordplay.
'cardinal say' becomes 'red' (cardinal is a shade of red).
'is back' shows that the letters should be reversed in order.
'holding' is an insertion indicator.
'red' backwards is 'der'.
'der' going around 'a' is 'DEAR'.
Can you help me to learn more?
(Other definitions for dear that I've seen before include "Baby" , "Beloved - expensive" , "close" , "letter opener" , "pet > ducks" .)