Pea trap forms barrier (7)
I believe the answer is:
parapet
'barrier' is the definition.
(thesaurus)
'pea trap forms' is the wordplay.
'forms' is an anagram indicator.
'pea'+'trap'='peatrap'
'peatrap' with letters rearranged gives 'PARAPET'.
(Other definitions for parapet that I've seen before include "Low wall along the side of a bridge" , "Low barrier edging balcony" , "Wall edging a roof or bridge" , "Low wall (on bridge, roof, etc)" , "protection from fire" .)