Spend too much time running to Guernsey to get away from veterinary surgeon (9)
I believe the answer is:
overtrain
'veterinary surgeon' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'spend too much time running to guernsey to get away from' is the wordplay.
I cannot quite understand how this works, but
'time' could be 't' (abbreviation) and 't' is present in the answer.
'away' could be 'a' (abbreviation as used in sports fixtures) and 'a' is present in the answer.
'from' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'too much' could be 'over' (I've seen this in other clues) and 'over' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for overtrain that I've seen before include "Prepare too hard" , "Spend too much time on exercises" , "Run past peak, perhaps" .)