The first old rocking Billy? (4)
I believe the answer is:
idol
'billy?' is the definition.
(I've seen this before)
'the first old rocking' is the wordplay.
'the first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'rocking' indicates anagramming the letters.
'old' anagrammed gives 'dol'.
'i'+'dol'='IDOL'
(Other definitions for idol that I've seen before include "'There's a one-eyed yellow -' (Hayes)" , "Graven image or worshipped person" , "Worshipped image; lido (anag.)" , "A matinee -- is a handsome actor" , "A graven image, or a hero" .)