Very funny situation includes a boy dressing (5,5)
I believe the answer is:
salad cream
'dressing' is the definition.
(I've seen this before)
'very funny situation includes a boy' is the wordplay.
'very funny situation' becomes 'scream' (scream can mean a hilarious situation).
'includes' is an insertion indicator.
'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" .)