Weep over unprotected Nero's grave (5)
I believe the answer is:
sober
'grave' is the definition.
(I've seen this before)
'weep over unprotected nero's' is the wordplay.
'weep' becomes 'sob' (sobbing is a kind of weeping).
'over' says to put letters next to each other.
'unprotected' says to take the centre.
The middle of 'nero' is 'er'.
'sob'+'er'='SOBER'
(Other definitions for sober that I've seen before include "Dignified -- lacking colour" , "Sensible - clear-headed" , "Far from drunk" , "Serious; muted in colour" , "Earnest" .)