Sad Castro's fourth cigar exploded (6)
I believe the answer is:
tragic
'sad' is the definition.
(similar in meaning)
'castro's fourth cigar exploded' is the wordplay.
'castro's fourth' becomes 't' (4th letter of 'castro').
'exploded' indicates anagramming the letters.
'cigar' is an anagram of 'ragic'.
't'+'ragic'='TRAGIC'
(Other definitions for tragic that I've seen before include "like King Lear" , "Greatly distressing" , "Calamitous, causing great sadness" , "Disastrous, desperately sad" , "Terrible" .)