Damage when river floods country (6)
I believe the answer is:
sprain
'damage' is the definition.
'river floods country' is the wordplay.
'river' becomes 'r'.
'floods' indicates putting letters inside (to flood can mean to fill up).
'country' becomes 'spain' (Spain is an example).
'r' placed into 'spain' is 'SPRAIN'.
'when' is the link.
(Other definitions for sprain that I've seen before include "Wrenching (of a muscle)" , "Wrench, the ankle say" , "Wrench (a ligament) (P, not T)" , "Injure a joint by twisting its ligaments" , "Muscular injury caused by wrench" .)