Doctor is able to descend by rope (6)
I believe the answer is:
abseil
'descend by rope' is the definition.
(I've seen this before)
'doctor is able' is the wordplay.
'doctor' indicates an anagram.
'is'+'able'='isable'
'isable' anagrammed gives 'ABSEIL'.
'to' acts as a link.
(Other definitions for abseil that I've seen before include "go down the line" , "Use ropes to climb down a cliff face" , "Descend down vertical surface using rope" , "going down" , "Climb down using a double rope" .)