Leaf midrib of lettuce thanks (5)
I believe the answer is:
costa
'leaf midrib' is the definition.
The definition suggests a singular noun which matches the answer.
'lettuce thanks' is the wordplay.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'cos'+'ta'='COSTA'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for costa that I've seen before include "-- Brava; -- Blanca" , "place for cheap holiday" , "A Spanish coastal region, perhaps del Sol" , "Rib or rib-like structure" , "A Spanish coast like Del Sol" .)