Ground supplied by river heading away from spring, perhaps (6)
I believe the answer is:
reason
'ground' is the definition.
(I know that ground can be written as reason)
'supplied by river heading away from spring perhaps' is the wordplay.
'supplied by' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'heading away from' means to remove the first letter.
'spring perhaps' becomes 'season' (spring is a season).
'season' with its initial letter taken off is 'eason'.
'r' next to 'eason' is 'REASON'.
(Other definitions for reason that I've seen before include "Cause; think logically" , "Capacity for rational thought" , "Mental power to justify an act" , "logical argument" , "Think in a logical way" .)