In Poitiers, go to get information on special substances that provoke reaction ... (9)

'substances that provoke reaction' is the definition.
'allergens' can be an answer for 'substances' (allergen is a kind of substance). I'm not certain of the rest of the definition.

'in poitiers go to get information on special' is the wordplay.
'in poitiers go' becomes 'aller' (I am not sure about this - if you are sure you should believe this answer much more).
'to get' says to put letters next to each other.
'information' becomes 'gen' (gen can informally mean information).
'on' is a charade indicator (letters next to each other).
'special' becomes 'S' (common abbreviation as in Special Air Service).
'aller'+'gen'+'s'='ALLERGENS'

Can you help me to learn more?