Saw magpie flying across river (7)
I believe the answer is:
epigram
'saw' is the definition.
(thesaurus)
'magpie flying across river' is the wordplay.
'flying' is an anagram indicator.
'across' means one lot of letters goes inside another.
'river' becomes 'r'.
'magpie' with letters rearranged gives 'epigam'.
'epigam' enclosing 'r' is 'EPIGRAM'.
(Other definitions for epigram that I've seen before include "short satirical poem" , "Concise witty remark" , "Clever remark" , "Brief witty saying" , "Gnome" .)