Form of halogen as source of salt (5)
I believe the answer is:
class
'form' is the definition.
(synonyms)
'halogen as source of salt' is the wordplay.
'halogen' becomes 'cl' (symbol for chlorine - chlorine is a halogen).
'source of' suggests taking the first letters.
The initial letter of 'salt' is 's'.
'cl'+'as'+'s'='CLASS'
'of' is the link.
(Other definitions for class that I've seen before include "People having the same social or economic status" , "Where one learns" , "26 [ORDER]" , "Standing" , "Body of students" .)