Monarch leaves grave to weep convulsively (3)
I believe the answer is:
sob
'weep convulsively' is the definition.
(synonyms)
'monarch leaves grave' is the wordplay.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'leaves' is a deletion indicator.
'grave' becomes 'sober' (synonyms).
'sober' with 'er' taken out is 'SOB'.
'to' acts as a link.
(Other definitions for sob that I've seen before include "Wep convulsively" , "Weep loudly" , "Cry noisily" , "Cry convulsively" , "Weep uncontrollably" .)