RESOLVED
Hello ! Does someone has a solution for this issue : the cloud sparse volume behind appears in front of the hairs ?
Thank you in advance for your help ! 
Edit : One brilliant man of the team found the solution ! 
Play with this : r.HairStrands.ComposeAfterTranslucency 2
and this if you have problem with DOF : r.HairStrands.DOFDepth 1
More informations about Rendering and Groom issues here
Rendering Hair & Fur