Insanity of crowd welcoming danger, oddly (7)
I believe the answer is:
madness
'insanity' is the definition.
(madness is a kind of insanity)
'crowd welcoming danger oddly' is the wordplay.
'crowd' becomes 'mass' (massing is a kind of crowding**).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'oddly' indicates alternate letters.
The alternating letters of 'danger' are 'dne'.
'mass' placed around 'dne' is 'MADNESS'.
'of' is the link.
(Other definitions for madness that I've seen before include "Absurdity" , "Insanity" , "Utter foolishness" , "Irrationality" , "Dementia" .)