The fourth Doctor W's memorial? (4)
I believe the answer is:
tomb
'memorial?' is the definition.
(I know that tomb is a type of memorial)
'the fourth doctor w's' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'fourth doctor w's' becomes 'omb' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'omb'='TOMB'
Can you help me to learn more?
(Other definitions for tomb that I've seen before include "Burial chamber" , "Sepulchral monument" , "Last resting-place" , "grave situation" , "Resting place" .)