Hoot catching a youngster dressing (5,5)
I believe the answer is:
salad cream
'dressing' is the definition.
(I've seen this before)
'hoot catching a youngster' is the wordplay.
'hoot' becomes 'scream' (I've seen this before**).
'catching' indicates putting letters inside.
'youngster' becomes 'lad' (I've seen this before).
'a'+'lad'='alad'
'scream' enclosing 'alad' is 'SALAD CREAM'.
(Other definitions for salad cream that I've seen before include "Sauce" , "Dressing like mayonnaise" .)