Trait shown by English pet tucking into meal with sauce (5,5,3)
I believe the answer is:
stiff upper lip
'trait shown by english' is the definition.
(term for stereotypical English stoicism)
'pet tucking into meal with sauce' is the wordplay.
'pet' becomes 'tiff' (I've seen this before).
'tucking into' is an insertion indicator.
'meal with' becomes 'supper' (I can't justify this - if you can you should believe this answer much more).
'sauce' becomes 'lip' (I've seen this before).
'tiff' going into 'supper' is 'stiffupper'.
'stiffupper'+'lip'='STIFF UPPER LIP'
(Other definitions for stiff upper lip that I've seen before include "Fortitude in adversity" , "Very British stoicism" , "It is kept by stoics" , "Evidence of stoicism" , "Uncomplaining stoicism needed" .)