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' indicates an anagram (letters rearranged).
'around' is an insertion indicator.
'mid' means to look at the middle letters (mid- can mean the middle of something).
'september' becomes 'sep' (short for September).
The middle letter of 'sep' is 'e'.
'tour' with letters rearranged gives 'outr'.
'outr' enclosing '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" .)