Practise too much in open water (9)
I believe the answer is:
overtrain
'practise too much' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'open water' is the wordplay.
'open' becomes 'overt' (synonyms).
'water' becomes 'rain' (rain is a kind of water).
'overt'+'rain'='OVERTRAIN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for overtrain that I've seen before include "Spend too much time on exercises" , "Prepare too hard" , "Run past peak, perhaps" .)