Band play, heading off across river (6)
I believe the answer is:
armlet
'band play heading off across river' is the definition.
I can't tell whether this definition defines the answer.
'band play' is the wordplay.
'band' becomes 'arm' (arm band is a kind of band).
'play' becomes 'let' (let is a kind of play).
'arm'+'let'='ARMLET'
(Other definitions for armlet that I've seen before include "branch" , "Band worn for decoration" , "Form of bracelet" , "Band for wearing" , "Type of bracelet" .)