Swindle involving river train (6)
I believe the answer is:
string
'train' is the definition.
(synonyms)
'swindle involving river' is the wordplay.
'swindle' becomes 'sting' (sting is a kind of swindle).
'involving' means one lot of letters goes inside another.
'river' becomes 'r'.
'sting' placed around 'r' is 'STRING'.
(Other definitions for string that I've seen before include "A lightweight cord" , "Cord, strand, thread" , "might one be plucked?" , "Violin wire" , "train" .)