Ridiculous day your old dad returned (8)
I believe the answer is:
derisory
'ridiculous' is the definition.
'day your old dad returned' is the wordplay.
'day' becomes 'd'.
'your' becomes 'yr'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dad' becomes 'sire' (sire can mean father).
'returned' says the letters should be written backwards.
'yr'+'o'+'sire'='yrosire'
'yrosire' reversed gives 'erisory'.
'd'+'erisory'='DERISORY'
(Other definitions for derisory that I've seen before include "Insultingly small" , "Ridiculously small" , "pathetically inadequate?" , "pathetically small" , "Contemptible" .)