Old lady sadly turning in: "so long and thanks for the fish dip" (12)
I believe the answer is:
taramasalata
'dip' is the definition.
(I've seen this before)
'old lady sadly turning in so long' is the wordplay.
'old lady' becomes 'rama' (I can't justify this - if you can you should believe this answer much more).
'sadly' becomes 'alas' ('alas' is used to express regret).
'turning' says the letters should be written in reverse.
'in' indicates putting letters inside.
'so long' becomes 'ta ta' (both are ways of saying goodbye).
'alas' backwards is 'sala'.
'rama'+'sala'='ramasala'
'ramasala' going within 'tata' is 'TARAMASALATA'.
'and thanks for the fish' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for taramasalata that I've seen before include "Greek paste made from fish roe" , "Greek fish pate or dip" , "Fish roe dip" , "fishy paste" , "Fishy Greek dip" .)
