Climber writes off first-class returns (8)
I believe the answer is:
wisteria
'climber' is the definition.
(the wisteria is a climbing plant)
'writes off first-class returns' is the wordplay.
'off' is an anagram indicator.
'first class' becomes 'ai' (A1 - top quality).
'returns' shows that the letters should be reversed in order.
'ai' in reverse letter order is 'ia'.
'writes' is an anagram of 'wister'.
'wister'+'ia'='WISTERIA'
(Other definitions for wisteria that I've seen before include "Climbing lilac-flowered shrub" , "I saw tire in this vine with the long flower clusters" , "As I write badly about the flowering vine" , "Vine with pendulous clusters of flowers" , "Large climbing shrub, often attached to house" .)