My posse occasionally laugh uncontrollably (6)
I believe the answer is:
corpse
'laugh uncontrollably' is the definition.
(as in corpsing during a theatrical performance)
'my posse occasionally' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'posse' are 'pse'.
'cor'+'pse'='CORPSE'
(Other definitions for corpse that I've seen before include "Dead bodies" , "Roof beam" , "Dead body" , "Cadaver, dead person" .)