Hiss at a pensioner on the box (4)
I believe the answer is:
soap
'the box' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'hiss at a pensioner on' is the wordplay.
'hiss at' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a pensioner' becomes 'OAP' (old age pensioner).
'on' says to put letters next to each other.
's'+'oap'='SOAP'
Can you help me to learn more?
(Other definitions for soap that I've seen before include "Cleaner - long-running TV show" , "Cleaner - TV programme" , "Serialised drama" , "TV entertainment" , "Cleansing substance" .)