Nothing held back by droll uncle (4)
I believe the answer is:
null
'nothing' is the definition.
(I know that nothing can be written as null)
'held back by droll uncle' is the wordplay.
'back by' is a reversal indicator.
'held' indicates a hidden word.
'llun' can be found hidden inside 'droll uncle'.
'llun' back-to-front is 'NULL'.
(Other definitions for null that I've seen before include "Invalid; empty" , "Without value or consequence" , "Useless - nothing" , "Legally void" , "Of no effect, value" .)