WC Grace finally caught by child — outstanding 150! (6)
I believe the answer is:
toilet
'wc' is the definition.
(a WC is a water closet or toilet)
'grace finally caught by child outstanding 150' is the wordplay.
'finally' suggests the final letters.
'caught by' indicates putting letters inside.
'child' becomes 'tot' (tot is a kind of child).
'outstanding' means one lot of letters goes inside another (some letters standing outside or around others).
't50' becomes 'IL' (Roman numerals for t and 50).
The last letter of 'grace' is 'e'.
'tot' going around 'il' is 'toilt'.
'e' placed inside 'toilt' is 'TOILET'.
(Other definitions for toilet that I've seen before include "It means washing" , "Waste disposal unit" , "WC" , "Plumbing fixture" , "something to go on?" .)