Says nothing to Tom, standing outside the shack (5,2)

'says nothing to' is the definition.
I can't judge whether this defines the answer.

'tom standing outside the shack' is the wordplay.
'tom' becomes 'puss' (I can't justify this - if you can you should believe this answer much more).
'standing' says the letters should be written backwards (in a down clue, letters stand up).
'outside' means one lot of letters goes inside another (some letters go outside others).
'the shack' becomes 'hut' ('hut' can be a synonym of 'shack').
'puss' in reverse letter order is 'ssup'.
'ssup' going around 'hut' is 'SHUTS UP'.

Can you help me to learn more?