Without sleep, role in a children's game is unsuitable (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(I've seen this before)
'without sleep role in a children's game' is the wordplay.
'without' is an insertion indicator.
'sleep' becomes 'nap' (a nap is a short sleep).
'role in a children's game' becomes 'it' ('you're it!').
'nap' inserted inside 'it' is 'INAPT'.
'is' acts as a link.
(Other definitions for inapt that I've seen before include "Far from fit" , "Improper" , "Not proper" , "Unsuited" , "Not fit, unsuitable" .)