Unsuitable cable held by mischievous child on river (8)
I believe the answer is:
improper
'unsuitable' is the definition.
(synonyms)
'cable held by mischievous child on river' is the wordplay.
'cable' becomes 'rope' (I've seen this before).
'held by' indicates putting letters inside.
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation).
'imp'+'r'='impr'
'rope' placed inside 'impr' is 'IMPROPER'.
(Other definitions for improper that I've seen before include "Unfitting, out of place" , "Not in line with accepted standards of morality" , "So it may appear a fraction" , "Not conforming to norms of conduct, maybe indecent" , "Not in accord with norms of behaviour" .)