Trait shown by English pet tucking into meal with sauce (5,5,3)
I believe the answer is:
stiff upper lip
Here is my best explanation:
'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'