Shout going topless is best (5)
I believe the answer is:
cream
'is best' is the definition.
(I've seen this before)
'shout going topless' is the wordplay.
'shout' becomes 'scream' (scream is a kind of shout).
'going topless' means to remove the first letter.
'scream' with its initial letter taken away is 'CREAM'.
(Other definitions for cream that I've seen before include "Best part of the milk" , "Butterfat part of milk" , "treat for cat?" , "It rises to the top, the best" , "Thick milk" .)