One's trapped in broken cattle grid (7)
I believe the answer is:
lattice
'grid' is the definition.
(thesaurus)
'one's trapped in broken cattle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'trapped in' indicates putting letters inside.
'broken' is an anagram indicator.
'cattle' with letters rearranged gives 'lattce'.
'i' placed within 'lattce' is 'LATTICE'.
(Other definitions for lattice that I've seen before include "Crystalline structure" , "Sort of window" , "Open framework in pattern" , "Framework of crossed laths or bars" , "Support for plants" .)