Run off to the doctor with much pain (5)
I believe the answer is:
drain
'run off' is the definition.
(I've seen this in another clue)
'the doctor with much pain' is the wordplay.
'the doctor' becomes 'd' (eg in PhD, Doctor of Philosophy).
'with' is a charade indicator (letters next to each other).
'much pain' becomes 'rain' (I can't explain this - if you can you should give a lot more credence to this answer).
'd'+'rain'='DRAIN'
'to' is the link.
Can you help me to learn more?
(Other definitions for drain that I've seen before include "Draw off U sewer" , "Exhaust - pipe" , "Exhaust (of liquid)" , "Wasted down here" , "Discharge pipe" .)