Mother returned father's second last letter in shock (6)
I believe the answer is:
amazed
'in shock' is the definition.
(I've seen this in another clue)
'mother returned father's second last letter' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'returned' is a reversal indicator.
'father's second' becomes 'a' (2nd letter of 'father').
'last letter' becomes 'zed' (in the alphabet).
'ma' written backwards gives 'am'.
'am'+'a'+'zed'='AMAZED'
(Other definitions for amazed that I've seen before include "Greatly surprised and impressed" , "Completely surprised" , "Filled with astonishment" , "was stunning" , "Greatly surprised or astonished" .)