A boy gets into something very funny, dressing (5,5)
I believe the answer is:
salad cream
'dressing' is the definition.
(I've seen this before)
'a boy gets into something very funny' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'gets into' indicates putting letters inside.
'something very funny' becomes 'scream' (I've seen this in another clue).
'a'+'lad'='alad'
'alad' put within 'scream' is 'SALAD CREAM'.
(Other definitions for salad cream that I've seen before include "Sauce" , "Dressing like mayonnaise" .)