Bit heads off three ancient deities (3)
I believe the answer is:
tad
'bit' is the definition.
(both can mean a small amount)
'heads off three ancient deities' is the wordplay.
'heads off' says to take the initial letters (the 'heads' of the words).
The first letters of 'three ancient deities' is 'TAD'.
(Other definitions for tad that I've seen before include "A small amount (colloq.)" , "Modest amount" , "Smidgen" , "Small amount or degree" , "little boy" .)