Laugh about a boy dressing (5,5)
I believe the answer is:
salad cream
'dressing' is the definition.
(I know that salad cream is a type of dressing)
'laugh about a boy' is the wordplay.
'laugh' becomes 'scream' (scream is a kind of laugh).
'about' means one lot of letters goes inside another.
'boy' becomes 'lad' (informal term for boy).
'a'+'lad'='alad'
'scream' placed around 'alad' is 'SALAD CREAM'.
(Other definitions for salad cream that I've seen before include "Dressing like mayonnaise" , "Sauce" .)