Temperature over Havana is dire (6)
I believe the answer is:
tragic
'dire' is the definition.
(I've seen this before)
'temperature over havana' is the wordplay.
'temperature' becomes 't' (abbreviation).
'over' shows that the letters should be reversed in order.
'havana' becomes 'cigar' (I've seen this before).
'cigar' reversed gives 'ragic'.
't'+'ragic'='TRAGIC'
'is' is the link.
(Other definitions for tragic that I've seen before include "Dire" , "Disastrous, desperately sad" , "Mournful" , "Grievous" , "Ill-fated" .)