Peripheral tour arranged around mid-September (5)
I believe the answer is:
outer
'peripheral' is the definition.
(I've seen this before)
'tour arranged around mid-september' is the wordplay.
'arranged' is an anagram indicator.
'around' means one lot of letters goes inside another.
'mid' indicates the central letters.
'september' becomes 'sep' ('Sep' can be a synonym of 'september').
The middle of 'sep' is 'e'.
'tour' is an anagram of 'outr'.
'outr' going around 'e' is 'OUTER'.
(Other definitions for outer that I've seen before include "Ring for double scores on a dartboard" , "Route (anag.)" , "External; peripheral" , "Archer's shot" , "Poor shot" .)