Absolutely ridiculous nite-bank? (8)
I believe the answer is:
entirely
'absolutely' is the definition.
(synonyms)
'ridiculous nite-bank?' is the wordplay.
'ridiculous' is an anagram indicator.
'bank?' becomes 'rely' (to bank on something is to rely on it).
'nite' anagrammed gives 'enti'.
'enti'+'rely'='ENTIRELY'
(Other definitions for entirely that I've seen before include "Wholly, totally" , "Wholly, completly" , "Wholly and completely" , "without qualification _" , "Without reservation" .)