Bottle first of port during function (5)
I believe the answer is:
spine
'bottle first' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'port during function' is the wordplay.
'port' becomes 'p'.
'during' means one lot of letters goes inside another.
'function' becomes 'sine' (mathematical function).
'p' placed inside 'sine' is 'SPINE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for spine that I've seen before include "Part of book's jacket" , "Part of a book" , "Vertebral column" , "Backbone" , "Line of vertebrae" .)