Model takes part at river mouth (5)
I believe the answer is:
delta
'river mouth' is the definition.
(I've seen this before)
'model takes part' is the wordplay.
'DELTA' is hidden in amongst the letters.
However, I am not sure how this is indicated.
'at' is the link.
Can you help me to learn more?
(Other definitions for delta that I've seen before include "Letter - where river divides" , "Dealt (anag.)" , "Triangular river system" , "River mouth in the Greek alphabet" , "Greek character" .)