Treatment centre using chlorine in one case primarily (6)
I believe the answer is:
clinic
'treatment centre' is the definition.
(I've seen this before)
'chlorine in one case primarily' is the wordplay.
'chlorine' becomes 'Cl' (Cl is the chemical symbol for chlorine).
'one' becomes 'i' (Roman numeral).
'primarily' indicates taking the first letters.
The initial letter of 'case' is 'c'.
'cl'+'in'+'i'+'c'='CLINIC'
'using' is the link.
(Other definitions for clinic that I've seen before include "Place for medical treatment or political advice" , "Place for specialised treatment or advice" , "Place for specialised treatment, from doctor or politician" , "Building where outpatients are treated" , "medical department" .)