Grand in cash, say, for mischievous child (6)
I believe the answer is:
monkey
I believe this is a double definition.
'grand in cash' is the first definition.
(informal term for a thousand pounds)
'for mischievous child' is the second definition.
(I've seen this before)
'say' acts as a link.
(Other definitions for monkey that I've seen before include "PS500 (sl.)" , "Mischievous child; primate" , "Type of wrench for a monkey" , "Chile pine (the Araucaria tree)" , "Primate with a long tail" .)