Quick look across river (4)
I believe the answer is:
spry
'quick' is the definition.
(synonyms)
'look across river' is the wordplay.
'look' becomes 'spy' ().
'across' is an insertion indicator.
'river' becomes 'r'.
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Sprightly" , "Alert" , "Fleet" , "Energetic" , "Nimble, brisk" .)