Young relative with £1,000 told off once for pinching pound (10)

'young relative' is the definition.
(young member of a family)

'ps1000 told off once for pinching pound' is the wordplay.
'psg000' becomes 'grand' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'told off once' becomes 'chid' (archaic past tense form of 'chide').
'for pinching' means one lot of letters goes inside another (some letters pinch or steal others).
'pound' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'chid' enclosing 'l' is 'child'.
'grand'+'child'='GRANDCHILD'

'with' is the link.