Climber in a fix on the ascent (5)
I believe the answer is:
liana
'climber' is the definition.
(climbing plant)
'a fix on the ascent' is the wordplay.
'fix' becomes 'nail' (I've seen this before).
'on the ascent' says the letters should be written backwards (in a down clue, letters go up).
'a'+'nail'='anail'
'anail' written backwards gives 'LIANA'.
'in' acts as a link.
(Other definitions for liana that I've seen before include "Climbing-plant" , "Woody climbing (usually tropical) plant" , "Climbing plant of rainforests" , "climber in the tropics" , "Tropical vine" .)