Field runs across river: it’s outstanding (6)
I believe the answer is:
arrear
'it's outstanding' is the definition.
I can't judge whether this definition defines the answer.
'field runs across river' is the wordplay.
'field' becomes 'area' (synonyms).
'runs' becomes 'r' (cricket abbreviation).
'across' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'area'+'r'='arear'
'arear' going around 'r' is 'ARREAR'.
Can you help me to learn more?
(Other definitions for arrear that I've seen before include "Backlog as of work" , "A rarer kind of backlog" , "not been done" , "Backward once" , "Amount unpaid or undone" .)