Children ultimately worry cattle (4)
I believe the answer is:
neat
'cattle' is the definition.
(thesaurus)
'children ultimately worry' is the wordplay.
'ultimately' says to take the final letters.
'worry' becomes 'eat' (eating is a kind of worrying**).
The last letter of 'children' is 'n'.
'n'+'eat'='NEAT'
(Other definitions for neat that I've seen before include "without ice?" , "Very good American" , "straight to the barmaid" , "farm animals" , "Spick and span" .)