Calamitous stunt mostly taking in a guide leader (6)
I believe the answer is:
tragic
'calamitous' is the definition.
(thesaurus)
'stunt mostly taking in a guide leader' is the wordplay.
'stunt' becomes 'trick' (I've seen this before).
'mostly' means to remove the last letter.
'taking in' is an insertion indicator.
'a guide leader' becomes 'ag' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trick' with its last letter taken away is 'tric'.
'tric' placed around 'ag' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Heart-rending" , "Greatly distressing" , "like King Lear" , "Mournful" , "Extremely sad and distressing" .)