Lively spot across river (4)
I believe the answer is:
spry
'lively' is the definition.
(I've seen this in another clue)
'spot across river' is the wordplay.
'spot' becomes 'spy' (synonyms).
'across' indicates putting letters inside.
'river' becomes 'r' (abbreviation).
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Nimble, brisk" , "'Active and lively, nimble (4)'" , "Light on one's feet" , "Still able to move lightly and fast" , "(Of an old person) lively" .)