Accustom one to being in the river (5)
I believe the answer is:
inure
'accustom' is the definition.
(I know that inure is a kind of accustom)
'one to being in the river' is the wordplay.
'one to being in' becomes 'in'.
'the river' becomes 'ure' (river in Yorkshire).
'in'+'ure'='INURE'
(Other definitions for inure that I've seen before include "Harden, habituate" , "To give experience" , "Steel" , "Temper, toughen" , "Get used to conditions" .)