Being realistic, cold wader left river (4-6)
I believe the answer is:
hard-bitten
'being realistic' is the definition.
(hard-bitten means realistic or unsentimental)
'cold wader left river' is the wordplay.
'cold' becomes 'hard' (both can mean unemotional).
'wader' becomes 'bittern' (wading bird).
'left' suggests deleting specific letters.
'river' becomes 'r' (abbreviation for river).
'bittern' with 'r' taken away is 'bitten'.
'hard'+'bitten'='HARD-BITTEN'
(Other definitions for hard-bitten that I've seen before include "Tough, cynical" , "Cynical and ruthless" .)