Heartless pranks on pensioner one picked on (3,4)
I believe the answer is:
joe soap
'one picked on' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'heartless pranks on pensioner' is the wordplay.
'heartless' suggests removing the centre.
'pranks' becomes 'jokes' ('joke' can be a synonym of 'prank').
'on' is a charade indicator (letters next to each other).
'pensioner' becomes 'oap'.
'jokes' with its centre taken out is 'joes'.
'joes'+'oap'='JOE SOAP'
Can you help me to learn more?
(Other definitions for joe soap that I've seen before include "scapegoat" , "He's just an ordinary man - but clean?" , "dopey sort" , "One saddled with nasty work" , "Fellow" .)