Mind one's held by both hands, a help to climbers (7)
I believe the answer is:
tendril
'a help to climbers' is the definition.
'mind one's held by both hands' is the wordplay.
'mind' becomes 'tend' (to tend is to look after or mind somebody).
'one' becomes 'i' (Roman numeral).
'held by' means one lot of letters goes inside another.
'both hands' becomes 'rl'.
'i' put inside 'rl' is 'ril'.
'tend'+'ril'='TENDRIL'
(Other definitions for tendril that I've seen before include "Climbing plant's supporter" , "Climbing plant's attachment" , "Clinging part of a plant" , "Slender thread-like appendage of a plant" , "Slender structure on some twining plants" .)