Part for Greta Garbo that's child's play (3)
I believe the answer is:
tag
'child's play' is the definition.
(I've seen this in another clue)
'part for greta garbo' is the wordplay.
'part' says the answer is hidden in the clue.
'TAG' can be found hidden inside 'for greta garbo'.
'that's' is the link.
(Other definitions for tag that I've seen before include "Children's chasing game" , "stock quotation" , "dog" , "Restriction" , "Earmark" .)