Wander off but don't go across river (5)
I believe the answer is:
stray
'wander off' is the definition.
(I've seen this before)
'don't go across river' is the wordplay.
'don't go' becomes 'stay' (I've seen this before).
'across' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'stay' enclosing 'r' is 'STRAY'.
'but' acts as a link.
(Other definitions for stray that I've seen before include "Wander from the path" , "Wander off course" , "out of place" , "Drift - isolated" , "Homeless cat" .)