Gastropod newsletter, sent the old-fashioned way (5,4)
I believe the answer is:
snail mail
'sent the old-fashioned way' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'gastropod newsletter' is the wordplay.
'gastropod' becomes 'snail' (snail is a kind of gastropod).
'newsletter' becomes 'mail' (I can't explain this - if you can you should believe this answer much more).
'snail'+'mail'='SNAIL MAIL'
Can you help me to learn more?
(Other definitions for snail mail that I've seen before include "Slow delivery" , "The conventional postal service" , "Postal system (facetious)" , "Modern jargon for letter post, mollusc-slow compared to electronic transport" , "Conventional (slow) post (informal)" .)