Thingummyjig had holes, 500, all around (6)
I believe the answer is:
doodah
'thingummyjig' is the definition.
I can't tell whether this defines the answer.
'had holes 500 all around' is the wordplay.
'holes' becomes 'OO' (two O's - O looks like a hole).
'500' becomes 'D' (Roman numeral for 500).
'all around' shows that the letters should be reversed in order.
'had'+'oo'+'d'='hadood'
'hadood' in reverse letter order is 'DOODAH'.
Can you help me to learn more?
(Other definitions for doodah that I've seen before include "Whatsit" , "Thingamy" , "Thingummy" , "thingumajig" , "one can't name" .)