Smoke rising beneath apartment's base: it's disastrous (6)
I believe the answer is:
tragic
'disastrous' is the definition.
(I've seen this before)
'smoke rising beneath apartment's base' is the wordplay.
'smoke' becomes 'cigar' (smoke can mean a cigar).
'rising' is a reversal indicator (letters written upwards in a down clue).
'beneath' says to put letters next to each other (in a down clue, some letters go below others).
'base' suggests the final letters (the base of the word).
The final letter of 'apartment' is 't'.
'cigar' written backwards gives 'ragic'.
'ragic' put after 't' is 'TRAGIC'.
'it's' is the link.
(Other definitions for tragic that I've seen before include "Disastrous, pitiful" , "Pitiable" , "Involving grief of death or destruction" , "Unfortunate" , "Calamitous, very sad" .)