Annihilate multitude about to catch a cold (8)
I believe the answer is:
massacre
'annihilate' is the definition.
(thesaurus)
'multitude about to catch a cold' is the wordplay.
'multitude' becomes 'mass' ('mass' can be a synonym of 'multitude').
'about' becomes 're' (regarding**).
'to catch' means one lot of letters goes inside another.
'cold' becomes 'c' (eg on taps).
'mass'+'re'='massre'
'a'+'c'='ac'
'massre' enclosing 'ac' is 'MASSACRE'.
(Other definitions for massacre that I've seen before include "Murder (of many persons)" , "Kill" , "Wanton slaughter" , "Heavily defeat" , "bloody victory" .)