Lost when river filters into another (6)
I believe the answer is:
astray
'lost' is the definition.
(synonyms)
'when river filters into another' is the wordplay.
I cannot really see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is present in the answer.
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
The remaining letters 'tay' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for astray that I've seen before include "Off the track - in error" , "Well off course" , "Off target" , "on the wrong road" , "(Gone) in wrong direction" .)