Corpulent man is calamitous (5)
I believe the answer is:
fatal
'calamitous' is the definition.
(thesaurus)
'corpulent man' is the wordplay.
'corpulent' becomes 'fat' ('fat' can be similar in meaning to 'corpulent').
'man' becomes 'al' (**).
'fat'+'al'='FATAL'
'is' is the link.
(Other definitions for fatal that I've seen before include "Leading to disaster" , "Causing death, say accident" , "Disastrous, maybe lethal" , "Unavoidable" , "Mortal" .)