Adders somehow spot letter-opener, presumably (9)
I believe the answer is:
addressee
'letter-opener presumably' is the definition.
I can't tell whether this definition defines the answer.
'adders somehow spot' is the wordplay.
'somehow' indicates anagramming the letters.
'spot' becomes 'see' (I've seen this in another clue).
'adders' with letters rearranged gives 'addres'.
'addres'+'see'='ADDRESSEE'
Can you help me to learn more?
(Other definitions for addressee that I've seen before include "He's written to" , "He gets a letter" , "Person named on an envelope" , "Recipient of post" , "one who's likely to get the post" .)