Portion at heart oddly peripheral (7)
I believe the answer is:
printer
'peripheral' is the definition.
(I know that printer is a type of peripheral)
'portion at heart oddly' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'portion'+'at'+'heart'='portionatheart'
The alternating letters of 'portionatheart' are 'PRINTER'.
(Other definitions for printer that I've seen before include "Producer of reading material" , "Computer attachment to produce paper documents" , "See 13 Down" , "Computer peripheral" , "who makes an impression" .)