Back in first part of post-lunch period (3)
I believe the answer is:
aft
'back' is the definition.
(associated in meaning)
'first part of post-lunch period' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'part of post lunch period' becomes 'ft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'ft'='AFT'
'in' is the link.
Can you help me to learn more?
(Other definitions for aft that I've seen before include "near the tail" , "Toward's a ship's stern" , "At the back at sea" , "behind boat" , "Towards the rear at sea" .)