Smear live fish ingesting uranium (6)
I believe the answer is:
bedaub
'smear' is the definition.
(synonyms)
'live fish ingesting uranium' is the wordplay.
'live' becomes 'be' (synonyms).
'fish' becomes 'dab' (dab is a type of fish).
'ingesting' indicates putting letters inside.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'dab' enclosing 'u' is 'daub'.
'be'+'daub'='BEDAUB'
(Other definitions for bedaub that I've seen before include "Crudely paint" , "Smear (with a sticky substance)" , "Smear with something sticky" , "Splodge with paint" .)