Fasten up a trailing plant (5)
I believe the answer is:
liana
'plant' is the definition.
The answer is an example.
'fasten up a trailing' is the wordplay.
'fasten' becomes 'nail' (I've seen this in another clue).
'up' is a reversal indicator (in down clue: letters go upwards).
'trailing' means one lot of letters go next to another.
'nail' written backwards gives 'lian'.
'lian'+'a'='LIANA'
(Other definitions for liana that I've seen before include "Tropical forest climber" , "Woody climbing (usually tropical) plant" , "Climbing jungle plant" , "Tropical vine" , "Climbing-plant" .)