Location upset former schoolfellow? It’ll have no effect on me (7)
I believe the answer is:
placebo
'it'll have no effect on me' is the definition.
I can't tell whether this defines the answer.
'location upset former schoolfellow?' is the wordplay.
'location' becomes 'place' (place is a kind of location).
'upset' is a reversal indicator.
'former schoolfellow?' becomes 'ob' (abbreviation for old boy).
'ob' reversed gives 'bo'.
'place'+'bo'='PLACEBO'
(Other definitions for placebo that I've seen before include "Medicine given solely for psychological effect" , "Prescription to soothe" , "Medicine working by psychology only" , "Inactive medicine" , "Ineffective substance given to humour patient" .)