Like returning boy, plaster may be French and saucy (5,8)
I believe the answer is:
salad dressing
'like returning boy plaster' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'may be french and saucy' is the wordplay.
'may be french' becomes 'salad' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'saucy' becomes 'dressing' (I've seen this before).
'salad'+'dressing'='SALAD-DRESSING'
Can you help me to learn more?
(Other definitions for salad dressing that I've seen before include "Mayonnaise, e.g." , "sauce" , "leaves having taken this coat?" , "Vinaigrette or mayonnaise, for example" , "saucy thing!" .)