Play with those people by river mouth (3,9)
I believe the answer is:
the mousetrap
'play' is the definition.
(I know that The Mousetrap is a play)
'those people by river mouth' is the wordplay.
'those people' becomes 'them' ('they' refers to a group of people).
'by' says to put letters next to each other.
'river' becomes 'ouse' (Yorkshire river).
'mouth' becomes 'trap' (informal term for mouth).
'them'+'ouse'+'trap'='THE MOUSETRAP'
'with' acts as a link.
(Other definitions for the mousetrap that I've seen before include "Murder mystery" , "Agatha Christie's West End long-runner" , "Long-running Agatha Christie play" .)