Taking most of the salad back with a regretful expression (4)
I believe the answer is:
alas
'expression' is the definition.
(I have seen 'Expression of regret' mean 'alas' so perhaps 'expression' could also mean 'alas')
'taking most of the salad back with a regretful' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
The remaining letters 'las' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for alas that I've seen before include "Cry of woe" , "Proper, respectable" , "Cry of regret" , "Sadly, unfortunately" , "Regretfully!" .)