New dog's taken off lead to play casually (6)
I believe the answer is:
noodle
'play casually' is the definition.
(to noodle is to improvise musically)
I cannot understand how the rest of the clue works.
(Other definitions for noodle that I've seen before include "Simple person - in the soup?" , "Foolish person or slangy head" , "Piece of pasta for foolish person?" , "Head - simpleton - found in soup" , "Thin pasta strip" .)