I swear it could be a climbing plant (8)
I believe the answer is:
wisteria
'a climbing plant' is the definition.
(I know that wisteria is a type of vine)
'i swear it could be' is the wordplay.
'could be' is an anagram indicator.
'i'+'swear'+'it'='iswearit'
'iswearit' anagrammed gives 'WISTERIA'.
(Other definitions for wisteria that I've seen before include "climbing plant" , "Climbing lilac-flowered shrub" , "Large climbing shrub, often attached to house" , "I saw tire in this vine with the long flower clusters" , "Climber" .)